282494
A lens is made of flint glass (refractive index \(=\) 1.5). When the lens is immersed in a liquid of refractive index 1.25 , the focal length :
1 increases by a factor of 1.25
2 increases by a factor of 2.5
3 increases by a factor of 1.2
4 decreases by a factor of 1.2
Explanation:
B: Given,
Refractive index of flint glass \(\left(\mu_{\mathrm{g}}\right)=1.5\)
Refractive index of liquid \(\left(\mu_{\mathrm{L}}\right)=1.25\)
\(\mu_{\mathrm{g}}=1.5, \mu_{\mathrm{L}}=1.25\)
Using lens maker formula -
\(\frac{1}{\mathrm{f}}=\left(\mu_{\mathrm{g}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
When it is immersed in liquid-
\(\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{\mathrm{u}_{\mathrm{g}}}{\mu_{\mathrm{L}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
Divide (i) \& (ii),
\(\begin{gathered}
\frac{\mathrm{f}^{\prime}}{\mathrm{f}}=\frac{\left(\mu_{\mathrm{g}}-1\right)}{\left(\mu_{\mathrm{g}} / \mu_{\mathrm{L}}-1\right)}=\frac{1.5-1}{\left(\frac{1.5}{1.25}-1\right)} \\
\mathrm{f}^{\prime}=\mathrm{f} \times\left(\frac{0.5 \times 1.25}{0.25}\right) \\
\mathrm{f}^{\prime}=\mathrm{f} \times 2.5
\end{gathered}\)
UPSEE - 2006
Ray Optics
282495
A wire mesh consisting of very small squares is viewed at a distance of \(8 \mathrm{~cm}\) through a magnifying converging lens of focal length a 10 \(\mathrm{cm}\), kept close to the eye. The magnification produced by the lens is :
1 5
2 8
3 10
4 20
Explanation:
A Given, \(\mathrm{u}=-8 \mathrm{~cm}, \mathrm{f}=10 \mathrm{~cm}\)
By using lens formula-
\(\begin{aligned}
\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\
\frac{1}{\mathrm{v}}+\frac{1}{8}=\frac{1}{10} \\
\frac{1}{\mathrm{v}}=\frac{1}{10}-\frac{1}{8}=-\frac{1}{40} \\
\mathrm{v}=-40 \mathrm{~cm}
\end{aligned}\)
Magnification \((\mathrm{m})=\frac{\mathrm{v}}{\mathrm{u}}=\frac{-40}{-8}=5\)
UPSEE - 2006
Ray Optics
282496
Which of the following is true for rays coming from infinity?
1 Two images are formed
2 Continuous image is formed between focal points of upper and lower lens
3 One image is formed
4 None of the above
Explanation:
A Lens is made up of two layers of different refractive indices, for a given wavelength of light it will have two focal length and will form two images at two different points as there are refractive indices. Focal length is related to refractive index by
\(\frac{1}{\mathrm{f}} \propto(\mu-1)\)
JCECE - 2007
Ray Optics
282497
If a convex lens of focal length \(75 \mathrm{~cm}\) and a concave lens of focal length \(50 \mathrm{~cm}\) are combined together, what will be their resulting power?
1 \(-6.6 \mathrm{D}\)
2 \(+0.66 \mathrm{D}\)
3 \(+6.6 \mathrm{D}\)
4 \(-0.66 \mathrm{D}\)
Explanation:
D: Given,
\(\begin{aligned}
\mathrm{f}_1=75 \mathrm{~cm}, \mathrm{f}_2=-50 \mathrm{~cm} \\
\mathrm{f}=\frac{\mathrm{f}_1 \cdot \mathrm{f}_2}{\mathrm{f}_1+\mathrm{f}_2}=\frac{75 \times(-50)}{75-50} \\
\mathrm{f}=150 \mathrm{~cm}
\end{aligned}\)
Power of lens, \(\mathrm{P}=\frac{100}{7}\)
\(\begin{aligned}
P=\frac{100}{-150}=\frac{-2}{3} \\
P=-0.66 D
\end{aligned}\)
UPSEE - 2005
Ray Optics
282500
The magnification of an image by a convex lens is positive only when the object is placed
1 at its focus \(\mathrm{F}\)
2 between \(\mathrm{F}\) and \(2 \mathrm{~F}\)
3 at \(2 \mathrm{~F}\)
4 between \(\mathrm{F}\) and optical centre
(e) beyond \(2 \mathrm{~F}\)
Explanation:
D: The magnification of an image by convex lens is positive only when the object is placed between focal point (f) and optical centre because it passes through another focus after refraction through the lens. Another ray of light from the object passes through the optical centre of the lens and thus as per the rule goes through the lens. Therefore both the rays are produced backwards so that they meet at a point to form an image \(A^{\prime} B^{\prime}\). This image is virtual and magnified image. Since this virtual image is a non-inverted image. Hence the magnification of such an image is positive. Thus, the object must be between focus and optical centre.
282494
A lens is made of flint glass (refractive index \(=\) 1.5). When the lens is immersed in a liquid of refractive index 1.25 , the focal length :
1 increases by a factor of 1.25
2 increases by a factor of 2.5
3 increases by a factor of 1.2
4 decreases by a factor of 1.2
Explanation:
B: Given,
Refractive index of flint glass \(\left(\mu_{\mathrm{g}}\right)=1.5\)
Refractive index of liquid \(\left(\mu_{\mathrm{L}}\right)=1.25\)
\(\mu_{\mathrm{g}}=1.5, \mu_{\mathrm{L}}=1.25\)
Using lens maker formula -
\(\frac{1}{\mathrm{f}}=\left(\mu_{\mathrm{g}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
When it is immersed in liquid-
\(\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{\mathrm{u}_{\mathrm{g}}}{\mu_{\mathrm{L}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
Divide (i) \& (ii),
\(\begin{gathered}
\frac{\mathrm{f}^{\prime}}{\mathrm{f}}=\frac{\left(\mu_{\mathrm{g}}-1\right)}{\left(\mu_{\mathrm{g}} / \mu_{\mathrm{L}}-1\right)}=\frac{1.5-1}{\left(\frac{1.5}{1.25}-1\right)} \\
\mathrm{f}^{\prime}=\mathrm{f} \times\left(\frac{0.5 \times 1.25}{0.25}\right) \\
\mathrm{f}^{\prime}=\mathrm{f} \times 2.5
\end{gathered}\)
UPSEE - 2006
Ray Optics
282495
A wire mesh consisting of very small squares is viewed at a distance of \(8 \mathrm{~cm}\) through a magnifying converging lens of focal length a 10 \(\mathrm{cm}\), kept close to the eye. The magnification produced by the lens is :
1 5
2 8
3 10
4 20
Explanation:
A Given, \(\mathrm{u}=-8 \mathrm{~cm}, \mathrm{f}=10 \mathrm{~cm}\)
By using lens formula-
\(\begin{aligned}
\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\
\frac{1}{\mathrm{v}}+\frac{1}{8}=\frac{1}{10} \\
\frac{1}{\mathrm{v}}=\frac{1}{10}-\frac{1}{8}=-\frac{1}{40} \\
\mathrm{v}=-40 \mathrm{~cm}
\end{aligned}\)
Magnification \((\mathrm{m})=\frac{\mathrm{v}}{\mathrm{u}}=\frac{-40}{-8}=5\)
UPSEE - 2006
Ray Optics
282496
Which of the following is true for rays coming from infinity?
1 Two images are formed
2 Continuous image is formed between focal points of upper and lower lens
3 One image is formed
4 None of the above
Explanation:
A Lens is made up of two layers of different refractive indices, for a given wavelength of light it will have two focal length and will form two images at two different points as there are refractive indices. Focal length is related to refractive index by
\(\frac{1}{\mathrm{f}} \propto(\mu-1)\)
JCECE - 2007
Ray Optics
282497
If a convex lens of focal length \(75 \mathrm{~cm}\) and a concave lens of focal length \(50 \mathrm{~cm}\) are combined together, what will be their resulting power?
1 \(-6.6 \mathrm{D}\)
2 \(+0.66 \mathrm{D}\)
3 \(+6.6 \mathrm{D}\)
4 \(-0.66 \mathrm{D}\)
Explanation:
D: Given,
\(\begin{aligned}
\mathrm{f}_1=75 \mathrm{~cm}, \mathrm{f}_2=-50 \mathrm{~cm} \\
\mathrm{f}=\frac{\mathrm{f}_1 \cdot \mathrm{f}_2}{\mathrm{f}_1+\mathrm{f}_2}=\frac{75 \times(-50)}{75-50} \\
\mathrm{f}=150 \mathrm{~cm}
\end{aligned}\)
Power of lens, \(\mathrm{P}=\frac{100}{7}\)
\(\begin{aligned}
P=\frac{100}{-150}=\frac{-2}{3} \\
P=-0.66 D
\end{aligned}\)
UPSEE - 2005
Ray Optics
282500
The magnification of an image by a convex lens is positive only when the object is placed
1 at its focus \(\mathrm{F}\)
2 between \(\mathrm{F}\) and \(2 \mathrm{~F}\)
3 at \(2 \mathrm{~F}\)
4 between \(\mathrm{F}\) and optical centre
(e) beyond \(2 \mathrm{~F}\)
Explanation:
D: The magnification of an image by convex lens is positive only when the object is placed between focal point (f) and optical centre because it passes through another focus after refraction through the lens. Another ray of light from the object passes through the optical centre of the lens and thus as per the rule goes through the lens. Therefore both the rays are produced backwards so that they meet at a point to form an image \(A^{\prime} B^{\prime}\). This image is virtual and magnified image. Since this virtual image is a non-inverted image. Hence the magnification of such an image is positive. Thus, the object must be between focus and optical centre.
282494
A lens is made of flint glass (refractive index \(=\) 1.5). When the lens is immersed in a liquid of refractive index 1.25 , the focal length :
1 increases by a factor of 1.25
2 increases by a factor of 2.5
3 increases by a factor of 1.2
4 decreases by a factor of 1.2
Explanation:
B: Given,
Refractive index of flint glass \(\left(\mu_{\mathrm{g}}\right)=1.5\)
Refractive index of liquid \(\left(\mu_{\mathrm{L}}\right)=1.25\)
\(\mu_{\mathrm{g}}=1.5, \mu_{\mathrm{L}}=1.25\)
Using lens maker formula -
\(\frac{1}{\mathrm{f}}=\left(\mu_{\mathrm{g}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
When it is immersed in liquid-
\(\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{\mathrm{u}_{\mathrm{g}}}{\mu_{\mathrm{L}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
Divide (i) \& (ii),
\(\begin{gathered}
\frac{\mathrm{f}^{\prime}}{\mathrm{f}}=\frac{\left(\mu_{\mathrm{g}}-1\right)}{\left(\mu_{\mathrm{g}} / \mu_{\mathrm{L}}-1\right)}=\frac{1.5-1}{\left(\frac{1.5}{1.25}-1\right)} \\
\mathrm{f}^{\prime}=\mathrm{f} \times\left(\frac{0.5 \times 1.25}{0.25}\right) \\
\mathrm{f}^{\prime}=\mathrm{f} \times 2.5
\end{gathered}\)
UPSEE - 2006
Ray Optics
282495
A wire mesh consisting of very small squares is viewed at a distance of \(8 \mathrm{~cm}\) through a magnifying converging lens of focal length a 10 \(\mathrm{cm}\), kept close to the eye. The magnification produced by the lens is :
1 5
2 8
3 10
4 20
Explanation:
A Given, \(\mathrm{u}=-8 \mathrm{~cm}, \mathrm{f}=10 \mathrm{~cm}\)
By using lens formula-
\(\begin{aligned}
\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\
\frac{1}{\mathrm{v}}+\frac{1}{8}=\frac{1}{10} \\
\frac{1}{\mathrm{v}}=\frac{1}{10}-\frac{1}{8}=-\frac{1}{40} \\
\mathrm{v}=-40 \mathrm{~cm}
\end{aligned}\)
Magnification \((\mathrm{m})=\frac{\mathrm{v}}{\mathrm{u}}=\frac{-40}{-8}=5\)
UPSEE - 2006
Ray Optics
282496
Which of the following is true for rays coming from infinity?
1 Two images are formed
2 Continuous image is formed between focal points of upper and lower lens
3 One image is formed
4 None of the above
Explanation:
A Lens is made up of two layers of different refractive indices, for a given wavelength of light it will have two focal length and will form two images at two different points as there are refractive indices. Focal length is related to refractive index by
\(\frac{1}{\mathrm{f}} \propto(\mu-1)\)
JCECE - 2007
Ray Optics
282497
If a convex lens of focal length \(75 \mathrm{~cm}\) and a concave lens of focal length \(50 \mathrm{~cm}\) are combined together, what will be their resulting power?
1 \(-6.6 \mathrm{D}\)
2 \(+0.66 \mathrm{D}\)
3 \(+6.6 \mathrm{D}\)
4 \(-0.66 \mathrm{D}\)
Explanation:
D: Given,
\(\begin{aligned}
\mathrm{f}_1=75 \mathrm{~cm}, \mathrm{f}_2=-50 \mathrm{~cm} \\
\mathrm{f}=\frac{\mathrm{f}_1 \cdot \mathrm{f}_2}{\mathrm{f}_1+\mathrm{f}_2}=\frac{75 \times(-50)}{75-50} \\
\mathrm{f}=150 \mathrm{~cm}
\end{aligned}\)
Power of lens, \(\mathrm{P}=\frac{100}{7}\)
\(\begin{aligned}
P=\frac{100}{-150}=\frac{-2}{3} \\
P=-0.66 D
\end{aligned}\)
UPSEE - 2005
Ray Optics
282500
The magnification of an image by a convex lens is positive only when the object is placed
1 at its focus \(\mathrm{F}\)
2 between \(\mathrm{F}\) and \(2 \mathrm{~F}\)
3 at \(2 \mathrm{~F}\)
4 between \(\mathrm{F}\) and optical centre
(e) beyond \(2 \mathrm{~F}\)
Explanation:
D: The magnification of an image by convex lens is positive only when the object is placed between focal point (f) and optical centre because it passes through another focus after refraction through the lens. Another ray of light from the object passes through the optical centre of the lens and thus as per the rule goes through the lens. Therefore both the rays are produced backwards so that they meet at a point to form an image \(A^{\prime} B^{\prime}\). This image is virtual and magnified image. Since this virtual image is a non-inverted image. Hence the magnification of such an image is positive. Thus, the object must be between focus and optical centre.
282494
A lens is made of flint glass (refractive index \(=\) 1.5). When the lens is immersed in a liquid of refractive index 1.25 , the focal length :
1 increases by a factor of 1.25
2 increases by a factor of 2.5
3 increases by a factor of 1.2
4 decreases by a factor of 1.2
Explanation:
B: Given,
Refractive index of flint glass \(\left(\mu_{\mathrm{g}}\right)=1.5\)
Refractive index of liquid \(\left(\mu_{\mathrm{L}}\right)=1.25\)
\(\mu_{\mathrm{g}}=1.5, \mu_{\mathrm{L}}=1.25\)
Using lens maker formula -
\(\frac{1}{\mathrm{f}}=\left(\mu_{\mathrm{g}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
When it is immersed in liquid-
\(\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{\mathrm{u}_{\mathrm{g}}}{\mu_{\mathrm{L}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
Divide (i) \& (ii),
\(\begin{gathered}
\frac{\mathrm{f}^{\prime}}{\mathrm{f}}=\frac{\left(\mu_{\mathrm{g}}-1\right)}{\left(\mu_{\mathrm{g}} / \mu_{\mathrm{L}}-1\right)}=\frac{1.5-1}{\left(\frac{1.5}{1.25}-1\right)} \\
\mathrm{f}^{\prime}=\mathrm{f} \times\left(\frac{0.5 \times 1.25}{0.25}\right) \\
\mathrm{f}^{\prime}=\mathrm{f} \times 2.5
\end{gathered}\)
UPSEE - 2006
Ray Optics
282495
A wire mesh consisting of very small squares is viewed at a distance of \(8 \mathrm{~cm}\) through a magnifying converging lens of focal length a 10 \(\mathrm{cm}\), kept close to the eye. The magnification produced by the lens is :
1 5
2 8
3 10
4 20
Explanation:
A Given, \(\mathrm{u}=-8 \mathrm{~cm}, \mathrm{f}=10 \mathrm{~cm}\)
By using lens formula-
\(\begin{aligned}
\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\
\frac{1}{\mathrm{v}}+\frac{1}{8}=\frac{1}{10} \\
\frac{1}{\mathrm{v}}=\frac{1}{10}-\frac{1}{8}=-\frac{1}{40} \\
\mathrm{v}=-40 \mathrm{~cm}
\end{aligned}\)
Magnification \((\mathrm{m})=\frac{\mathrm{v}}{\mathrm{u}}=\frac{-40}{-8}=5\)
UPSEE - 2006
Ray Optics
282496
Which of the following is true for rays coming from infinity?
1 Two images are formed
2 Continuous image is formed between focal points of upper and lower lens
3 One image is formed
4 None of the above
Explanation:
A Lens is made up of two layers of different refractive indices, for a given wavelength of light it will have two focal length and will form two images at two different points as there are refractive indices. Focal length is related to refractive index by
\(\frac{1}{\mathrm{f}} \propto(\mu-1)\)
JCECE - 2007
Ray Optics
282497
If a convex lens of focal length \(75 \mathrm{~cm}\) and a concave lens of focal length \(50 \mathrm{~cm}\) are combined together, what will be their resulting power?
1 \(-6.6 \mathrm{D}\)
2 \(+0.66 \mathrm{D}\)
3 \(+6.6 \mathrm{D}\)
4 \(-0.66 \mathrm{D}\)
Explanation:
D: Given,
\(\begin{aligned}
\mathrm{f}_1=75 \mathrm{~cm}, \mathrm{f}_2=-50 \mathrm{~cm} \\
\mathrm{f}=\frac{\mathrm{f}_1 \cdot \mathrm{f}_2}{\mathrm{f}_1+\mathrm{f}_2}=\frac{75 \times(-50)}{75-50} \\
\mathrm{f}=150 \mathrm{~cm}
\end{aligned}\)
Power of lens, \(\mathrm{P}=\frac{100}{7}\)
\(\begin{aligned}
P=\frac{100}{-150}=\frac{-2}{3} \\
P=-0.66 D
\end{aligned}\)
UPSEE - 2005
Ray Optics
282500
The magnification of an image by a convex lens is positive only when the object is placed
1 at its focus \(\mathrm{F}\)
2 between \(\mathrm{F}\) and \(2 \mathrm{~F}\)
3 at \(2 \mathrm{~F}\)
4 between \(\mathrm{F}\) and optical centre
(e) beyond \(2 \mathrm{~F}\)
Explanation:
D: The magnification of an image by convex lens is positive only when the object is placed between focal point (f) and optical centre because it passes through another focus after refraction through the lens. Another ray of light from the object passes through the optical centre of the lens and thus as per the rule goes through the lens. Therefore both the rays are produced backwards so that they meet at a point to form an image \(A^{\prime} B^{\prime}\). This image is virtual and magnified image. Since this virtual image is a non-inverted image. Hence the magnification of such an image is positive. Thus, the object must be between focus and optical centre.
282494
A lens is made of flint glass (refractive index \(=\) 1.5). When the lens is immersed in a liquid of refractive index 1.25 , the focal length :
1 increases by a factor of 1.25
2 increases by a factor of 2.5
3 increases by a factor of 1.2
4 decreases by a factor of 1.2
Explanation:
B: Given,
Refractive index of flint glass \(\left(\mu_{\mathrm{g}}\right)=1.5\)
Refractive index of liquid \(\left(\mu_{\mathrm{L}}\right)=1.25\)
\(\mu_{\mathrm{g}}=1.5, \mu_{\mathrm{L}}=1.25\)
Using lens maker formula -
\(\frac{1}{\mathrm{f}}=\left(\mu_{\mathrm{g}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
When it is immersed in liquid-
\(\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{\mathrm{u}_{\mathrm{g}}}{\mu_{\mathrm{L}}}-1\right)\left(\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right)\)
Divide (i) \& (ii),
\(\begin{gathered}
\frac{\mathrm{f}^{\prime}}{\mathrm{f}}=\frac{\left(\mu_{\mathrm{g}}-1\right)}{\left(\mu_{\mathrm{g}} / \mu_{\mathrm{L}}-1\right)}=\frac{1.5-1}{\left(\frac{1.5}{1.25}-1\right)} \\
\mathrm{f}^{\prime}=\mathrm{f} \times\left(\frac{0.5 \times 1.25}{0.25}\right) \\
\mathrm{f}^{\prime}=\mathrm{f} \times 2.5
\end{gathered}\)
UPSEE - 2006
Ray Optics
282495
A wire mesh consisting of very small squares is viewed at a distance of \(8 \mathrm{~cm}\) through a magnifying converging lens of focal length a 10 \(\mathrm{cm}\), kept close to the eye. The magnification produced by the lens is :
1 5
2 8
3 10
4 20
Explanation:
A Given, \(\mathrm{u}=-8 \mathrm{~cm}, \mathrm{f}=10 \mathrm{~cm}\)
By using lens formula-
\(\begin{aligned}
\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\
\frac{1}{\mathrm{v}}+\frac{1}{8}=\frac{1}{10} \\
\frac{1}{\mathrm{v}}=\frac{1}{10}-\frac{1}{8}=-\frac{1}{40} \\
\mathrm{v}=-40 \mathrm{~cm}
\end{aligned}\)
Magnification \((\mathrm{m})=\frac{\mathrm{v}}{\mathrm{u}}=\frac{-40}{-8}=5\)
UPSEE - 2006
Ray Optics
282496
Which of the following is true for rays coming from infinity?
1 Two images are formed
2 Continuous image is formed between focal points of upper and lower lens
3 One image is formed
4 None of the above
Explanation:
A Lens is made up of two layers of different refractive indices, for a given wavelength of light it will have two focal length and will form two images at two different points as there are refractive indices. Focal length is related to refractive index by
\(\frac{1}{\mathrm{f}} \propto(\mu-1)\)
JCECE - 2007
Ray Optics
282497
If a convex lens of focal length \(75 \mathrm{~cm}\) and a concave lens of focal length \(50 \mathrm{~cm}\) are combined together, what will be their resulting power?
1 \(-6.6 \mathrm{D}\)
2 \(+0.66 \mathrm{D}\)
3 \(+6.6 \mathrm{D}\)
4 \(-0.66 \mathrm{D}\)
Explanation:
D: Given,
\(\begin{aligned}
\mathrm{f}_1=75 \mathrm{~cm}, \mathrm{f}_2=-50 \mathrm{~cm} \\
\mathrm{f}=\frac{\mathrm{f}_1 \cdot \mathrm{f}_2}{\mathrm{f}_1+\mathrm{f}_2}=\frac{75 \times(-50)}{75-50} \\
\mathrm{f}=150 \mathrm{~cm}
\end{aligned}\)
Power of lens, \(\mathrm{P}=\frac{100}{7}\)
\(\begin{aligned}
P=\frac{100}{-150}=\frac{-2}{3} \\
P=-0.66 D
\end{aligned}\)
UPSEE - 2005
Ray Optics
282500
The magnification of an image by a convex lens is positive only when the object is placed
1 at its focus \(\mathrm{F}\)
2 between \(\mathrm{F}\) and \(2 \mathrm{~F}\)
3 at \(2 \mathrm{~F}\)
4 between \(\mathrm{F}\) and optical centre
(e) beyond \(2 \mathrm{~F}\)
Explanation:
D: The magnification of an image by convex lens is positive only when the object is placed between focal point (f) and optical centre because it passes through another focus after refraction through the lens. Another ray of light from the object passes through the optical centre of the lens and thus as per the rule goes through the lens. Therefore both the rays are produced backwards so that they meet at a point to form an image \(A^{\prime} B^{\prime}\). This image is virtual and magnified image. Since this virtual image is a non-inverted image. Hence the magnification of such an image is positive. Thus, the object must be between focus and optical centre.
