Online Test: Optics | Class 12 Physics

Question 1 of 20

The focal length of a concave mirror of radius of curvature $30\,\text{cm}$ is:

$30\,\text{cm}$

$15\,\text{cm}$

$60\,\text{cm}$

$7.5\,\text{cm}$

Explanation: $f = R/2 = 30/2 = 15\,\text{cm}$.

Question 2 of 20

A convex mirror forms an image that is always:

real and inverted

real and magnified

inverted and magnified

virtual, erect, diminished

Explanation: A convex mirror gives a virtual, erect, diminished image for all real objects.

Question 3 of 20

Magnification of a mirror is given by:

$m = v/u$

$m = -v/u$

$m = u/v$

$m = uv$

Explanation: For mirrors $m = -v/u = h\u2032/h$.

Question 4 of 20

Under the sign convention, distances measured against the incident light are taken as:

positive

zero

imaginary

negative

Explanation: Incident light travels left to right (positive); the opposite direction is negative.

Question 5 of 20

When the object is at the centre of curvature of a concave mirror, $m$ equals:

$+1$

$-1$

$+2$

$-2$

Explanation: Image at $C$, same size and inverted, so $m = -1$.

Question 6 of 20

Light bends away from the normal when entering a:

denser medium

medium of same index

vacuum

rarer medium

Explanation: Speed increases in a rarer medium, so the ray bends away from the normal.

Question 7 of 20

The critical angle for a medium of refractive index $2$ (with air) is:

$30^\circ$

$45^\circ$

$60^\circ$

$90^\circ$

Explanation: $\sin i_c = 1/2 \Rightarrow i_c = 30^\circ$.

Question 8 of 20

The power of a lens of focal length $25\,\text{cm}$ is:

$+2\,\text{D}$

$+0.25\,\text{D}$

$+25\,\text{D}$

$+4\,\text{D}$

Explanation: $P = 1/0.25 = +4\,\text{D}$.

Question 9 of 20

For a biconvex lens the radii are taken as:

both positive

both negative

$R_1$ positive, $R_2$ negative

$R_1$ negative, $R_2$ positive

Explanation: In the lens maker\u2019s convention $R_1 > 0$ and $R_2 < 0$ for a biconvex lens.

Question 10 of 20

The lens formula is:

$\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}$

$\dfrac{1}{u}-\dfrac{1}{v}=\dfrac{1}{f}$

$v+u=f$

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$

Explanation: For a thin lens $\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$.

Question 11 of 20

At minimum deviation through a prism, the refracted ray inside the prism is:

parallel to the base

perpendicular to the base

along the base

undefined

Explanation: At $\delta_m$ the ray passes symmetrically, travelling parallel to the base.

Question 12 of 20

In an astronomical telescope, high magnification needs:

large $f_e$, small $f_o$

equal $f_o$ and $f_e$

large $f_o$, small $f_e$

zero $f_e$

Explanation: $M = f_o/f_e$, so a large objective and small eyepiece focal length maximise $M$.

Question 13 of 20

Huygens\u2019 principle is used to explain:

photoelectric effect

reflection and refraction of waves

radioactivity

thermionic emission

Explanation: The wavelet construction derives the laws of reflection and refraction.

Question 14 of 20

In Young\u2019s experiment, bright fringes occur where the path difference is:

$(n+\tfrac{1}{2})\lambda$

$\lambda/4$

$n\lambda$

zero only

Explanation: Constructive interference needs $\Delta = n\lambda$.

Question 15 of 20

Fringe width $\beta = \dfrac{\lambda D}{d}$. If $d$ is doubled, $\beta$ becomes:

doubled

halved

unchanged

four times

Explanation: $\beta \propto 1/d$, so doubling $d$ halves $\beta$.

Question 16 of 20

The central maximum in single-slit diffraction is:

twice as wide as side maxima

equal in width to side maxima

narrower than side maxima

dark

Explanation: The central maximum spans between the first minima on each side, twice the width of the side maxima.

Question 17 of 20

Malus\u2019 law gives the intensity through an analyser as:

$I_0\sin\theta$

$I_0\cos^2\theta$

$I_0\sin^2\theta$

$I_0\cos\theta$

Explanation: $I = I_0\cos^2\theta$, where $\theta$ is the angle between polariser and analyser axes.

Question 18 of 20

Unpolarised light of intensity $I_0$ through one Polaroid emerges with intensity:

$I_0/2$

$I_0$

$I_0/4$

zero

Explanation: Averaging $\cos^2\theta$ over all angles gives $1/2$.

Question 19 of 20

An object at the focus of a concave mirror forms its image at:

the focus

the pole

infinity

the centre of curvature

Explanation: Rays after reflection become parallel, so the image forms at infinity.

Question 20 of 20

Total internal reflection is used in:

optical fibres

plane mirrors

concave lenses

photographic film

Explanation: Optical fibres guide light by repeated total internal reflection.

Online Test — Optics