Light and Optics Practice Questions
20 free Light and Optics practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following phenomena is responsible for the twinkling of stars?
- A Atmospheric reflection
- B Total internal reflection
- C Atmospheric refraction
- D Dispersion of light
Correct answer: Atmospheric refraction
Twinkling of stars is caused by the atmospheric refraction of starlight. As starlight passes through different layers of the atmosphere with varying refractive indices, the apparent position of the star fluctuates.
When light travels from an optically denser medium to a rarer medium, how does its speed and direction change?
- A Speed decreases and it bends towards the normal.
- B Speed increases and it bends away from the normal.
- C Speed increases and it bends towards the normal.
- D Speed decreases and it bends away from the normal.
Correct answer: Speed increases and it bends away from the normal.
In a rarer medium, light faces less resistance and travels faster. According to Snell's Law, when moving to a medium with a lower refractive index, the light ray bends away from the normal line.
What type of image is always formed by a convex mirror, regardless of the object's distance?
- A Real, inverted, and magnified
- B Real, upright, and diminished
- C Virtual, inverted, and diminished
- D Virtual, upright, and diminished
Correct answer: Virtual, upright, and diminished
Convex mirrors are diverging mirrors. They always produce a virtual image that is upright (erect) and smaller than the actual object, which is why they are used as rear-view mirrors in vehicles.
The splitting of white light into its constituent colors when passing through a glass prism is known as:
- A Refraction
- B Dispersion
- C Diffraction
- D Interference
Correct answer: Dispersion
Dispersion occurs because different colors of light travel at different speeds in glass, causing them to bend by different angles. Violet light bends the most, while red light bends the least.
A person suffering from myopia (near-sightedness) requires which type of lens for correction?
- A Convex lens
- B Concave lens
- C Cylindrical lens
- D Bifocal lens
Correct answer: Concave lens
In myopia, the eyeball is too long or the lens too curved, causing images to focus in front of the retina. A concave (diverging) lens is used to spread the light rays so the image focuses correctly on the retina.
What is the critical angle for a medium if the light undergoes total internal reflection when hitting the boundary with a vacuum?
- A The angle of incidence for which the angle of refraction is 0°.
- B The angle of incidence for which the angle of refraction is 90°.
- C The angle of incidence that is equal to the angle of reflection.
- D The angle at which light is completely absorbed by the medium.
Correct answer: The angle of incidence for which the angle of refraction is 90°.
The critical angle is the specific angle of incidence that results in an angle of refraction of 90 degrees. Any incidence angle greater than the critical angle results in total internal reflection.
Which part of the human eye controls the amount of light entering the eye by adjusting the size of the pupil?
- A Cornea
- B Retina
- C Iris
- D Lens
Correct answer: Iris
The iris is the colored part of the eye that acts like a shutter. It contains muscles that contract or expand the pupil to regulate light exposure to the retina.
In a compound microscope, the intermediate image formed by the objective lens is:
- A Virtual, upright, and magnified
- B Real, upright, and diminished
- C Real, inverted, and magnified
- D Virtual, inverted, and magnified
Correct answer: Real, inverted, and magnified
The objective lens of a microscope forms a real, inverted, and enlarged image of the object. This intermediate image then acts as the object for the eyepiece lens.
What is the power of a lens with a focal length of +25 cm?
- A +0.25 Dioptre
- B +4.0 Dioptre
- C -4.0 Dioptre
- D +2.5 Dioptre
Correct answer: +4.0 Dioptre
Power (P) is the reciprocal of focal length (f) in meters. Since 25 cm is 0.25 m, P = 1 / 0.25 = +4.0 D.
Which of the following is an application of total internal reflection?
- A Blue color of the sky
- B Formation of a rainbow
- C Optical fiber communication
- D Working of a magnifying glass
Correct answer: Optical fiber communication
Optical fibers use total internal reflection to transmit light signals over long distances with minimal loss. The light reflects repeatedly off the inner walls of the fiber core.
A red rose appears black when viewed under pure green light. Why does this happen?
- A The rose reflects only green light and absorbs all other colors.
- B The rose absorbs green light and has no red light to reflect.
- C Green light is complementary to red and neutralizes it.
- D The green light is refracted away from the observer's eye.
Correct answer: The rose absorbs green light and has no red light to reflect.
An object's color is determined by the light it reflects. A red rose absorbs almost all colors except red; since green light contains no red, the rose absorbs the green light and reflects nothing, appearing black.
Which mirror is used by dentists to see a large image of a patient's teeth?
- A Plane mirror
- B Convex mirror
- C Concave mirror
- D Two-way mirror
Correct answer: Concave mirror
When an object is placed very close to a concave mirror (between the pole and focus), it produces a virtual, upright, and magnified image. This allows dentists to see a detailed view of the teeth.
The phenomenon of interference of light proves that light has:
- A Wave nature
- B Particle nature
- C Transverse nature only
- D Dual nature
Correct answer: Wave nature
Interference, along with diffraction, is a characteristic behavior of waves. It occurs when two or more light waves overlap to form a resultant wave of greater, lower, or the same amplitude.
According to the Cartesian sign convention, the focal length of a concave mirror is always:
- A Negative
- B Positive
- C Zero
- D Infinite
Correct answer: Negative
In the Cartesian sign convention, distances measured in the direction of incident light are positive. Since the focus of a concave mirror is in front of the reflecting surface (opposite to the incident light direction), it is considered negative.
What is the primary cause of 'Mirage' observed in hot deserts?
- A Total internal reflection due to temperature gradients
- B Equal heating of all layers of air above the sand
- C Scattering of sunlight by suspended sand particles
- D Diffraction of light passing close to the hot desert floor
Correct answer: Total internal reflection due to temperature gradients
A mirage is an optical illusion where light rays are bent via refraction through layers of air with different temperatures. Near the ground, the air is very hot (low density), causing light to undergo total internal reflection.
The refractive index of diamond is 2.42. What does this value imply about the speed of light in diamond?
- A Light travels 2.42 times faster in diamond than in vacuum.
- B Light travels at the same speed as in vacuum.
- C Light travels 2.42 times slower in diamond than in vacuum.
- D The speed of light in diamond is 2.42 x 10⁸ m/s.
Correct answer: Light travels 2.42 times slower in diamond than in vacuum.
Refractive index (n) is the ratio of the speed of light in a vacuum (c) to the speed in the medium (v). Therefore, v = c / 2.42, meaning light travels 2.42 times slower in diamond.
Where should an object be placed in front of a convex lens so that a real image of the same size is formed?
- A At the focus (F)
- B At infinity
- C Between the focus and the optical center
- D At twice the focal length (2F)
Correct answer: At twice the focal length (2F)
When an object is placed at 2F in front of a convex lens, the image is formed at 2F on the other side. This image is real, inverted, and exactly the same size as the object.
The 'blue color' of the clear sky is primarily due to:
- A Refraction of light
- B Reflection of the ocean
- C Polarization of light
- D Rayleigh scattering
Correct answer: Rayleigh scattering
Rayleigh scattering occurs when sunlight interacts with molecules in the atmosphere. Shorter wavelengths (blue and violet) are scattered much more efficiently than longer wavelengths (red).
Which property of light remains constant when it passes from one medium to another?
- A Wavelength
- B Velocity
- C Amplitude
- D Frequency
Correct answer: Frequency
Frequency is a property of the light source itself. While the speed and wavelength change as the refractive index of the medium changes, the number of wave crests passing a point per second remains constant.
What is the focal length of a plane mirror?
- A Infinite
- B Zero
- C Equal to the object distance
- D Depends on the size of the mirror
Correct answer: Infinite
A plane mirror is considered a spherical mirror with an infinite radius of curvature. Since the focal length is half the radius of curvature (f = R/2), the focal length of a plane mirror is also infinite.