Ray Optics Class 12 Important Questions with Answers

Ans. By using lens makers formula (i) If $n_{1}>n_{2}$ then $f$ will be positive. It means that lens will show opposite nature $i . e .$ it will converge the light rays (ii) If $n_{1}=n_{2}$ then $f$ will be infinite. It means that lens behaves like a plane glass plate i.e.n light raysn passes undeviated. (iii) If $n_{1}

Ans. By using lens makers formula $\quad \frac{1}{f}=\left(\frac{\mu_{2}}{\mu_{1}}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ Where $\mu_{1}=n_{1}=$ refractive index of medium $\mu_{2}=n_{2}=$ refractive index of lens Hence $\frac{1}{f}=\left(\frac{n_{1}}{n_{2}}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ (i) If $n_{1}>n_{2}$ then $f$ will be positive. It means thatlens will show normal behaviour i.e. it will converge the light rays (ii) If $n_{1}=n_{2}$ then $f$ will be infinite. It means that lens behaves like a plane glass plate i.e. light rays pass undeviated. (iii) If $n_{1}

Ans. In the position of minimum deviation $i=e, r_{1}=r_{2}=r$ Hence by using $i+e=A+\delta \Rightarrow i+i=A+D_{m}$ $\Rightarrow i=\frac{A+D_{m}}{2}$ …(i) Also $A=r_{1}+r_{2}=r+r=2 r \Rightarrow r=\frac{A}{2}$ …(ii) Hence by using snell's $\operatorname{law} \mu=\frac{\sin i}{\sin r} \Rightarrow \mu=\frac{\sin \frac{A+D_{m}}{2}}{\sin A / 2}$

Ans. For a real image (on wall), minimum distance between the object and image should be $4 f .$ $\therefore 4 f=3 m \quad \therefore f=\frac{3}{4} m=0.75 m$

Ans. Here, $\mu=1.55, R_{1}=R$ and $R_{2}=-R, f=20 \mathrm{cm}$ As $\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ $\therefore \frac{1}{20}=(1.55-1)\left(\frac{1}{R}+\frac{1}{R}\right)=\frac{1.10}{R}$$\Rightarrow \quad R=20 \times 1.1=22 \mathrm{cm}$

Ans. (a) Ray diagram (b) Magnification : Area of each square (i.e. object) $O=1 \mathrm{mm}^{2}$ Distance of object $u=-9 \mathrm{cm}$ Focal length of lens $f=+10 \mathrm{cm}$ By using lens formula $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ $\frac{1}{10}=\frac{1}{v}-\frac{1}{(-9)} \Rightarrow \frac{1}{v}=\frac{1}{10}-\frac{1}{9}=-\frac{1}{90} \Rightarrow v=-90 \mathrm{cm}$ Hence magnification $=\frac{v}{u}=\frac{90}{9}=10$ also $m^{2}=\frac{A_{\text {image }}}{A_{\text {object }}} \Rightarrow A_{\text {image}}=m^{2} \times A_{\text {object}}$ $=(10)^{2} \times 1 m m^{2}=100 m m^{2}$ (c) Magnification power $=\frac{D}{4}=\frac{25}{9}=2.8=2.8$

Ans. By using lens makers formula For the lens in air, $\mu_{2}=1.6, \mu_{1}=1,$ and $f=+20 \mathrm{cm}$

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 When lens is immersed in a liquid of $\mu=1.8$ then $\mu_{l e n s}<\mu_{m e d i u m} \mathrm{SO}$ lens behaves oppositely i.e like a diverging lens.

Ans. Here, size of candle, $h_{1}=2.5 \mathrm{cm}, u=-27 \mathrm{cm}, R=-36 \mathrm{cm}$ $\quad f=\frac{R}{2}=-18 \mathrm{cm}, v=?,$ As $\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$ $\therefore \frac{1}{v}=\frac{1}{f}-\frac{1}{u}=\frac{1}{-18}+\frac{1}{27}=\frac{-3+2}{54}=\frac{1}{54}$ or $v=-54 \mathrm{cm}$ $\therefore$ Screen should be placed at $54 \mathrm{cm}$ from the mirror on the same side as the object. If $h_{2}$ is size of image, then as the image is real $m=\frac{-h_{2}}{h_{1}}=\frac{-v}{-u}=\frac{v}{u} \Rightarrow \frac{-h_{2}}{2.5}=\frac{(-54)}{(-27)}=2$ $\Rightarrow h_{2}=-5 \mathrm{cm} .$ Minus sign indicates that image is inverted. When the candle is moved closer to mirror, the screen has to be moved away from the mirror. However, when candle is at a distance less than 18cm from the mirror, image formed would be virtual and screen is not required as image cannot be taken on the screen.

Ans. Here $h_{1}=3 c m, u=-14 c m, f=-21 c m, v=?$ As $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{-21}+\frac{1}{-14}=\frac{-5}{42} \Rightarrow v=\frac{-42}{5}=-8.4 \mathrm{cm}$ $\therefore$ Image is erect, virtual and at $8.4 \mathrm{cm}$ from the lens on the same side of the object. As $\frac{I}{O}=\frac{v}{u} \Rightarrow \frac{I}{3}=\frac{-8.4}{-14} \Rightarrow I=0.6 \times 3=1.8 \mathrm{cm}$ As the object is moved away from the lens, virtual image moves towards focus of lens (but never beyond focus). The size of image goes on decreasing.

Ans. Since light rays incident normally on face ab, so no refraction takes place and light rays passes undeviated and then incident on face ac at an angle of $49^{\circ}$. For total internal reflection at $a c$ $i>c \Rightarrow \sin i>\sin c \Rightarrow \sin 45^{\circ}>\frac{1}{\mu}$ $\Rightarrow \frac{1}{\sqrt{2}}>\frac{1}{\mu} \Rightarrow \mu>\sqrt{2}$ or $\mu>1.41$ As $\mu_{R}<\mu$ but $\mu_{G}>\mu$ and $\mu_{B}>\mu,$ so only red colour will be transmitted through face $a c$ while green and blue while internally reflected.

Ans. (a) For convex lens $u=+12 \mathrm{cm}, f=+20 \mathrm{cm}$ $\therefore \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \Rightarrow \frac{1}{20}=\frac{1}{v}-\frac{1}{12} \Rightarrow \frac{1}{20}+\frac{1}{12} \Rightarrow v=7.5 \mathrm{cm}$ Thus the beam converges at a point $7.5 \mathrm{cm}$ to the right of the lens. (b) For concave lens $u=+12 c m, f=-16 c m$ $\therefore \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \Rightarrow \frac{1}{-16}=\frac{1}{v}-\frac{1}{12} \Rightarrow \frac{1}{v}=\frac{-1}{16}+\frac{1}{12}$ $\Rightarrow v=48 c m$ Thus the beam converges at a point 48 cm to the right of the lens.

Ans. A convex lens is made up of two convex spherical refracting surfaces. The final image is formed after two refractions. In figure (A) $P_{1}, P_{2}$ are the poles, of a thin biconvex lens with optical centre at $O$. Let $\mu_{2}$ be the refractive index of material of lens and $\mu_{1}$ be the refractive index of the surrounding. Consider a point object $O^{\prime}$ lying on the principle axis of the lens. The first refracting surface $A P_{1} B$ forms the image $I_{1}$ of the object at a distance $v_{1}$ from the surface. The image $I_{1}$ acts as a virtual object for the second surface that forms the final image at $I$. The general equation for refraction at spherical surface is $\frac{\mu_{2}}{\text { distance of image }}-\frac{\mu_{1}}{\text { distance of object }}$ $=\frac{\mu_{2}-\mu_{1}}{\text { Radius of curvature }}$ …(i) To use this equation for the two surfaces $\left(A P_{1} B\right.$ and $\left.A P_{2} B\right)$ origin's are at $P_{1}$ and $P_{2}$ respectively. We should take the origin at O, because lens is Thin Refraction at first surface $A P_{1} B:$ For the first refraction object is at $O^{\prime}$ the image is at $I_{1}$ and centre of curvature of $C_{1}$. If $u, v_{1}$ and $R$ denotes their respective distance from $O$ then by using equation ( i ) $\frac{\mu_{2}}{v_{1}}-\frac{\mu_{1}}{u}=\frac{\mu_{2}-\mu_{2}}{R_{1}}$ …(ii) Refraction at second surface $A P_{2} B:$ For the second refraction from surface $A P_{2} B, I_{1}$ acts as virtual object, $I$ is the final image and $C_{2}$ is the centre of curvature of this surface. If $v_{1}, v$ and $R_{2}$ denotes object distance, and image distance and radius of curvature for this surface then again by using equation ( i) $\frac{1 / \mu_{2}}{v}-\frac{1 / \mu_{1}}{v_{1}}=\frac{\mu_{2}-\mu_{2}}{R_{1}}$ (Here $1 / \mu$ is taken in place of because refraction is taking place from denser to rarer medium) $\Rightarrow \frac{\mu_{1}}{v}-\frac{\mu_{2}}{v_{1}}=\frac{\mu_{1}-\mu_{2}}{R_{2}}=-\frac{\left(\mu_{2}-\mu_{1}\right)}{R_{2}} \ldots$ (iii) on adding equation (ii) and (iii) we get $\frac{\mu_{1}}{v}-\frac{\mu_{1}}{u}=\mu_{2}-\mu_{1}\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ …(ii) If $\mu_{1}=1$ and $\mu_{2}=\mu$ then $\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ this is the required result Assumptions : (i) The lens is thin so that distances measured from the poles of it’s surface can be taken as equal to the distance from the optical centre of the lens (ii) The aperture of the lens is small. (iii) The object is point object (iv) The incident ray and refracted ray makes small angles with the principle axis of the lens. Sign convention : (i) All distances are measured from the optical centre of the lens (ii) In the direction of light all the distances are taken positive while opposite to the direction of light all the distances are taken negative. (iii) Above the principal axis all the distances are taken positive while below the principal axis all distances are taken negative.