15 extra multiple-choice questions for Ray Optics (12th Standard Physics, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
In a plane mirror reflection, if the incident ray makes an angle of \(30^{\circ}\) with the mirror surface, what is the angle of deviation produced?
- A. \(30^{\circ}\)
- B. \(60^{\circ}\)Correct
- C. \(120^{\circ}\)
- D. \(150^{\circ}\)
Explanation. The angle of incidence is \(90^{\circ}\) minus the \(30^{\circ}\) glancing angle, which is \(60^{\circ}\). The deviation is \(180^{\circ}\) minus twice the angle of incidence, resulting in sixty degrees.
Q2
A convex mirror used in a vehicle has a radius of curvature of 40 cm. What is the focal length of this mirror according to the Cartesian sign convention?
- A. -20 cm
- B. +20 cmCorrect
- C. -40 cm
- D. +40 cm
Explanation. The focal length is half the radius of curvature. For a convex mirror, the focus is behind the mirror, so the focal length is positive under the sign convention.
Q3
An object is placed at a distance of 20 cm from a concave mirror with a focal length of 10 cm. What is the lateral magnification produced by the mirror?
- A. -1Correct
- B. +1
- C. -2
- D. +2
Explanation. Using the mirror formula, the image is formed at the center of curvature, which is 20 cm from the pole. The magnification is the negative ratio of image to object distance.
Q4
If the speed of light in a certain transparent oil is \(2.4 \times 10^8\) m s\(^{-1}\), what is the refractive index of the oil? (Assume speed of light in vacuum is \(3 \times 10^8\) m s\(^{-1}\))
- A. 0.8
- B. 1.2
- C. 1.25Correct
- D. 1.5
Explanation. Refractive index is defined as the ratio of the speed of light in vacuum to its speed in the medium. Dividing three by two point four gives one point two five.
Q5
A glass plate has a thickness of 5 cm and a refractive index of 1.6. What is the optical path of light passing through this glass plate?
- A. 3.125 cm
- B. 5.0 cm
- C. 8.0 cmCorrect
- D. 10.0 cm
Explanation. The optical path is the product of the physical distance and the refractive index of the medium. Multiplying five centimetres by one point six yields eight centimetres.
Q6
A ray of light is incident at \(60^{\circ}\) on the surface of a liquid with a refractive index of \(\sqrt{3}\). What is the angle of refraction?
- A. \(30^{\circ}\)Correct
- B. \(45^{\circ}\)
- C. \(60^{\circ}\)
- D. \(90^{\circ}\)
Explanation. According to Snell's law, the sine of the angle of incidence equals the refractive index times the sine of the angle of refraction. Calculation reveals the refracted angle to be thirty degrees.
Q7
At sunrise or sunset, the sun is visible before it actually crosses the horizon. Which phenomenon is responsible for this?
- A. Total internal reflection
- B. Atmospheric refractionCorrect
- C. Rayleigh scattering
- D. Diffraction
Explanation. Light from the sun passes through layers of air with gradually increasing density as it approaches Earth, causing it to bend towards the normal and appear higher than its actual position.
Q8
Under what condition will light travelling from one medium to another undergo total internal reflection?
- A. Rarer to denser medium, incidence angle less than critical
- B. Denser to rarer medium, incidence angle less than critical
- C. Denser to rarer medium, incidence angle greater than criticalCorrect
- D. Rarer to denser medium, incidence angle greater than critical
Explanation. Total internal reflection occurs when light moves from an optically denser medium to a rarer one at an angle of incidence that exceeds the specific critical angle for that interface.
Q9
In an optical fibre, if the refractive index of the core is \(n_1\) and the refractive index of the cladding is \(n_2\), the numerical aperture in air is given by:
- A. \(n_1^2 - n_2^2\)
- B. \(\sqrt{n_1^2 - n_2^2}\)Correct
- C. \(\frac{n_1}{n_2}\)
- D. \(\sqrt{n_1^2 + n_2^2}\)
Explanation. The numerical aperture determines the acceptance angle of the fibre. It is derived from Snell's law and the condition for total internal reflection at the core-cladding boundary.
Q10
A point object in air (\(n_1=1\)) is placed 30 cm from a convex spherical refracting surface made of plastic (\(n_2=2\)) with a radius of curvature of 15 cm. Where is the final image formed?
- A. 30 cm from the surface
- B. 45 cm from the surface
- C. 60 cm from the surfaceCorrect
- D. 90 cm from the surface
Explanation. Using the refraction formula for single spherical surfaces, substituting the object distance, radius, and refractive indices determines the image location sixty centimetres away on the other side.
Q11
A biconvex lens is manufactured from glass of refractive index 1.5. If the radii of curvature of the two surfaces are 20 cm and 30 cm, what is the focal length of the lens?
- A. 12 cm
- B. 24 cmCorrect
- C. 40 cm
- D. 60 cm
Explanation. By applying the lens maker's formula with the given refractive index and radii of curvature, the focal length is calculated to be twenty-four centimetres.
Q12
Two thin lenses of focal lengths +20 cm and -25 cm are placed in contact. What is the power of this combination in dioptres?
- A. -1 D
- B. +1 DCorrect
- C. -9 D
- D. +9 D
Explanation. The total power is the sum of individual powers. Power is calculated as the reciprocal of the focal length in metres, yielding five and negative four dioptres respectively.
Q13
When a prism is in the position of minimum deviation, which of the following is true for the refracted ray inside the prism?
- A. It is parallel to the base for an equilateral prismCorrect
- B. It travels along the normal
- C. It is perpendicular to the base
- D. It undergoes total internal reflection
Explanation. At minimum deviation, the ray passes symmetrically through the prism. For an equilateral or isosceles prism, this means the internal ray is parallel to the base.
Q14
A prism material has refractive indices of 1.66 and 1.62 for violet and red light respectively. If the mean refractive index is 1.64, calculate its dispersive power.
- A. 0.025
- B. 0.040
- C. 0.0625Correct
- D. 0.080
Explanation. Dispersive power is the ratio of the difference in extreme refractive indices to the mean refractive index minus one. The calculation yields zero point zero six two five.
Q15
Which property of light is responsible for the blue appearance of the sky?
- A. Longer wavelengths are scattered more
- B. Scattering intensity is independent of wavelength
- C. Shorter wavelengths are scattered moreCorrect
- D. Only blue light is reflected by air molecules
Explanation. According to Rayleigh scattering, the intensity of scattered light is inversely proportional to the fourth power of the wavelength. Blue light has a shorter wavelength and scatters more dominantly.
