Free Optics MCQs with Answers

11. The image formed by a plane mirror is

• A. real, inverted and the same size
• B. virtual, upright, laterally inverted and the same size as the object
• C. real, upright and magnified
• D. virtual and diminished

Explanation: The rays only appear to come from behind the mirror, so the image cannot be caught on a screen and is virtual, and it lies as far behind the mirror as the object is in front. Lateral inversion is why writing held up to a mirror reads backwards. The magnification is exactly one.

12. A concave mirror is used as a shaving or make up mirror because when the face is closer than the focal length it forms an image that is

• A. real and inverted
• B. virtual, upright and magnified
• C. virtual and diminished
• D. real and diminished

Explanation: Inside the focal length the reflected rays diverge, so they appear to come from an enlarged upright image behind the mirror. Beyond the focal length the same mirror gives a real inverted image, which is how a reflecting telescope forms its image. A convex mirror always gives a diminished upright virtual image, which is why it is used for wide angle car mirrors.

13. The critical angle for a medium of refractive index 1.5 is closest to

• A. 30 degrees
• B. 42 degrees
• C. 60 degrees
• D. 90 degrees

Explanation: The sine of the critical angle equals one over the refractive index, so sin C is 0.667 and C is about 42 degrees, the standard value for ordinary glass. A higher refractive index gives a smaller critical angle, which is why diamond at 2.42 has a critical angle of only 24 degrees and traps light so effectively. Water, at 1.33, gives about 49 degrees.

14. The resolving power of a telescope is increased by

• A. increasing the magnification only
• B. increasing the diameter of its objective lens or mirror
• C. using a longer eyepiece
• D. using light of longer wavelength

Explanation: Resolution is limited by diffraction at the aperture, so a larger objective produces a smaller diffraction disc and separates closer objects, which is the main reason research telescopes are built so large. Magnification without resolution simply enlarges a blur. A shorter wavelength also improves resolution, which is why ultraviolet and electron imaging outperform visible light.

15. Light of wavelength 500 nm in a vacuum enters glass of refractive index 1.5. Inside the glass its wavelength becomes

• A. 750 nm
• B. 333 nm
• C. 500 nm
• D. 250 nm

Explanation: Frequency is unchanged on entering a medium, and since the speed falls by the factor of the refractive index the wavelength must fall in the same proportion, giving 500 divided by 1.5, which is about 333 nm. The colour perceived does not change, because it is determined by frequency. Multiplying instead gives the 750 nm distractor.

16. Diffraction of light becomes noticeable when the size of the obstacle or aperture is

• A. much larger than the wavelength
• B. comparable to the wavelength of the light
• C. much smaller than an atom
• D. irrelevant to the wavelength

Explanation: Light bends appreciably round an obstacle only when the obstacle is of the same order of size as its wavelength, and since visible wavelengths are under a micrometre this rarely happens with everyday objects. Sound, with wavelengths of about a metre, diffracts round doorways easily, which is why you can hear round a corner but not see round it. A diffraction grating exploits slits of the right scale.

17. In a compound microscope, the objective lens forms an image that is

• A. virtual and diminished
• B. real, inverted and magnified, which the eyepiece then magnifies further
• C. real and diminished
• D. at infinity

Explanation: The specimen sits just beyond the objective's focal point, so the objective produces a real magnified image inside the tube, and the eyepiece acts as a magnifying glass on that image. The total magnification is the product of the two, which is why a 40 times objective with a 10 times eyepiece gives 400 times. Both lenses are converging and of short focal length.

18. The sky appears blue because

• A. the atmosphere is blue in colour
• B. shorter wavelengths are scattered more strongly by air molecules
• C. the sea reflects onto the sky
• D. the Sun emits mostly blue light

Explanation: Rayleigh scattering varies as the inverse fourth power of wavelength, so blue is scattered far more than red and reaches the eye from all directions. At sunset the light travels a longer path through the atmosphere, so most of the blue is scattered away and the transmitted light appears red. The Sun's own output peaks in the green yellow region.

19. Polaroid sunglasses reduce glare because reflected light from a horizontal surface is

• A. partially polarised in the horizontal plane, which the filter blocks
• B. of a different frequency
• C. travelling faster
• D. already absorbed by the surface

Explanation: Reflection off water or a road partially polarises the light horizontally, so a filter oriented to transmit only vertical vibrations removes most of the glare while passing ordinary light. This works only because light is a transverse wave. Rotating the glasses through 90 degrees makes the glare reappear, which is a simple demonstration of the effect.

20. The least distance of distinct vision for a normal human eye is about

• A. 5 cm
• B. 25 cm
• C. 100 cm
• D. infinity

Explanation: About 25 cm is the closest an object can be brought while remaining in sharp focus without strain, and it is the reference distance used in the magnifying power formula for a lens. The near point recedes with age as the eye lens stiffens, which is why reading glasses are often needed after forty. The far point of a normal eye is at infinity.