Ray Optics and Optical Instruments — Class 12 MCQs with Answers

Ray Optics and Optical Instruments — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Snell's law:

   ANoneBn₁ + n₂ = θCn = c/v onlyDn₁ sinθ₁ = n₂ sinθ₂
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   Answer: n₁ sinθ₁ = n₂ sinθ₂

   Explanation: Snell's law governs refraction: n₁sinθ₁ = n₂sinθ₂, relating the incident and refracted angles through each medium's refractive index — it is not a sum of indices or n=c/v alone.

2. Q2 of 6Easy

   A simple microscope uses:

   AA single convex lens of short focal lengthBTwo concave lensesCA plane mirrorDA prism
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   Answer: A single convex lens of short focal length

   Explanation: A simple microscope is just one convex lens of short focal length held close to the eye, magnifying without the second lens a compound microscope needs.

3. Q3 of 6Medium

   Total internal reflection occurs when:

   ALight passes from denser to rarer beyond critical angleBAt normal incidenceCAlwaysDIn vacuum
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   Answer: Light passes from denser to rarer beyond critical angle

   Explanation: Total internal reflection needs light going from a denser to a rarer medium at an angle beyond the critical angle — normal incidence or a rarer-to-denser path never triggers it.

4. Q4 of 6Medium

   Assertion (A): In a compound microscope, the image formed by the objective lies near the focal point of the eyepiece. Reason (R): In practice, it is difficult to make the focal length of a lens much smaller than 1 cm.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: the first image is formed at or just within the focal plane of the eyepiece, so the eyepiece can act as a magnifier and give the final image at infinity or the near point. R is true, but it is about a practical limit on focal length, not about where the first image is placed, so it does not explain A.

5. Q5 of 6Hard

   A prism splits white light because refractive index:

   AIs zeroBIs constantCEquals 1DVaries with wavelength (dispersion)
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   Answer: Varies with wavelength (dispersion)

   Explanation: A prism splits white light because its refractive index varies with wavelength (dispersion): violet bends more than red since n is higher at shorter wavelengths.

6. Q6 of 6Hard

   Assertion (A): A convex lens of focal length 5 cm, used as a simple microscope with the final image at the near point (25 cm), gives a magnification of 6. Reason (R): A compound microscope can give a much larger magnification than a simple microscope.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: with the image at the near point, m = 1 + D/f = 1 + 25/5 = 6. R is true, since a simple microscope is limited to small magnifications, but it compares two instruments and says nothing about the value in A, so it does not explain A.

Ray Optics and Optical Instruments notes: key concepts (Class 12 Physics)

Light treated as rays: reflection at spherical mirrors with the mirror formula and sign convention, refraction and Snell's law, total internal reflection with its applications, refraction at a spherical surface leading to the lens maker's formula, lens power and combinations, the prism, and the microscope and telescope.

Laws of reflection

The angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal all lie in one plane.

Spherical mirrors

A concave mirror converges light and can form real or virtual images; a convex mirror always diverges and forms only virtual, diminished images.

Sign convention

Distances are measured from the pole, positive along the incident light and negative against it. Nearly every optics error traces back to a sign here.

Mirror formula and magnification

One relation links object distance, image distance and focal length, with magnification −v/u telling you the size and orientation of the image.

Refraction

The bending of light as it crosses into a medium of different optical density, caused entirely by the change in the SPEED of light.

Snell's law

The ratio of the sines of the angles of incidence and refraction is constant, equal to the ratio of refractive indices of the two media.

Refractive index

The ratio of the speed of light in vacuum to that in the medium. It is always greater than one and depends on the wavelength of light.

Total internal reflection

When light travels from a denser to a rarer medium beyond the critical angle, it is completely reflected — no light is refracted at all.

Critical angle

The angle of incidence in the denser medium for which the refracted ray just grazes the surface at 90°. Its sine equals the reciprocal of the refractive index.

Applications of TIR

Optical fibres, the sparkle of a diamond, mirages and the totally reflecting prisms in binoculars all rely on total internal reflection.

Refraction at a spherical surface

Relates object and image distances across a single curved boundary and is the step from which the lens maker's formula is derived.

Lens maker's formula

Gives the focal length from the refractive index and the two radii of curvature, showing how a lens's power depends on both shape and material.

Thin lens formula and magnification

Links object distance, image distance and focal length; the magnification v/u gives the image size and whether it is erect or inverted.

Power of a lens

The reciprocal of focal length in metres, measured in dioptre. Converging lenses have positive power, diverging lenses negative.

Combination of lenses

For thin lenses in contact, powers simply add algebraically — the basis of correcting lenses and of achromatic doublets.

Optical instruments

A simple microscope magnifies with a single converging lens; a compound microscope and a telescope each use an objective and an eyepiece, but with very different focal-length choices.