All questions of Light for UPSC CSE Exam

The correct answer is option 'A': Plane mirror.

A plane mirror is used in a car's rearview mirror. Let's understand why a plane mirror is chosen for this purpose.

1. Definition of a plane mirror:
- A plane mirror is a flat and smooth reflective surface that reflects light rays uniformly.
- It forms a virtual image that is upright, the same size as the object, and located behind the mirror.

2. Purpose of a rearview mirror in a car:
- The rearview mirror in a car is positioned inside the vehicle, usually on the windshield or the interior roof of the car.
- It allows the driver to see the reflection of the area behind the car without turning their head or diverting their attention from the road ahead.
- It helps the driver to monitor the traffic behind the car and make informed decisions while driving, such as changing lanes or reversing.

3. Advantages of using a plane mirror in a rearview mirror:
- Wide field of view: A plane mirror provides a wide field of view, allowing the driver to see a larger area behind the car.
- No distortion: Plane mirrors reflect light rays without distorting them, providing an accurate representation of the objects behind the car.
- Upright image: The image formed by a plane mirror is upright, which means it appears the same way as the actual object. This helps the driver perceive the distance and position of the objects accurately.
- Same size image: The image formed in a plane mirror is the same size as the object being reflected. This allows the driver to gauge the relative sizes of objects behind the car.

4. Comparison with other mirror types:
- Convex mirror: Convex mirrors are commonly used as side mirrors in cars. They provide a wider field of view but result in a smaller, distorted image. Convex mirrors are preferred for side mirrors as they help eliminate blind spots.
- Concave mirror: Concave mirrors are used in headlights and fog lights of cars to focus and direct the light. They are not suitable for rearview mirrors as they produce a magnified, inverted image.
- Spherical mirror: The term "spherical mirror" is a general term that includes both convex and concave mirrors. Since a plane mirror is a specific type of spherical mirror, it is the correct answer for the given question.

In conclusion, a plane mirror is used in a car's rearview mirror because it provides a wide, undistorted field of view, reflects an upright and same-sized image, and allows the driver to accurately perceive objects behind the car.

Luminous Intensity Definition
Luminous intensity is a measure of the perceived power of light emitted by a source in a particular direction. It quantifies how much light is emitted per unit solid angle.
Unit of Luminous Intensity
The correct unit for measuring luminous intensity is the candela (cd). Here’s why it is the right choice:

• Definition: One candela is defined as the luminous intensity of a source emitting monochromatic radiation of frequency 540 x 10^12 hertz and having a radiant intensity of 1/683 watt per steradian.
• Directionality: Unlike other units, candela specifically accounts for the direction of light emission, which is crucial for applications in lighting design and safety.
• SI Unit: The candela is one of the seven base units in the International System of Units (SI), highlighting its importance in scientific measurements.

Comparative Units
To understand how candela differs from other units related to light:

• Lux: This measures illuminance, or the amount of light hitting a surface, which is not the same as luminous intensity.
• Lumen: This unit measures the total amount of visible light emitted by a source, regardless of direction.
• Watt: This measures power, not specifically related to light perception or intensity.

Conclusion
In summary, the candela is the proper unit for luminous intensity, as it effectively describes how bright a light source appears in a given direction, making it essential for various applications in lighting and safety standards.

Understanding Convex Lenses
A convex lens is a crucial optical component known for its ability to converge light rays. Here’s a detailed explanation of its characteristics:
Definition of Convex Lens
- A convex lens is thicker at the center and thinner at the edges.
- It is shaped in such a way that it bulges outward, resembling a magnifying glass.
Light Convergence
- The primary function of a convex lens is to converge incoming parallel light rays to a single point, known as the focal point.
- As light passes through the lens, it is refracted (bent) towards the optical axis, causing it to focus.
Applications of Convex Lenses
- Convex lenses are widely used in various optical devices:
- Cameras: To focus light and create clear images.
- Magnifying glasses: To enlarge images for better visibility.
- Eyeglasses: To correct hyperopia (farsightedness) by converging light onto the retina.
Comparison with Other Lenses
- Unlike concave lenses, which are thicker at the edges and diverge light, convex lenses are designed to bring light together.
- Plano-convex lenses are a variation where one side is flat and the other is convex, still retaining the converging property.
- Plano-concave lenses are the opposite, causing light to diverge.
Conclusion
In conclusion, a convex lens (option B) is defined by its thicker center and its ability to converge light. This makes it a fundamental element in many optical applications, enhancing visibility and image clarity.

Understanding Light Refraction
When light travels from a denser medium, like water or glass, to a rarer medium, such as air, it undergoes a phenomenon known as refraction.
What Happens During Refraction?
- When light enters a denser medium, it slows down due to the medium's higher optical density.
- Conversely, when it moves to a rarer medium, it speeds up. This change in speed is crucial for understanding light behavior.
Why Does Light Speed Up?
- Optical Density: The denser the medium, the slower light travels through it. This is due to the interaction between photons and the atoms in the medium.
- Transition to Rarer Medium: Upon entering a rarer medium, the interaction decreases, allowing light to travel faster.
Real-Life Examples
- Air and Water: When light exits water into air, it accelerates, causing the light to bend away from the normal line (the imaginary line perpendicular to the surface).
- Visual Effects: This speed change causes phenomena like the bending of a straw in a glass of water, creating an optical illusion.
Conclusion
In summary, as light transitions from a denser medium to a rarer medium, it indeed speeds up. This fundamental concept of physics explains various optical phenomena we encounter in daily life. Understanding this principle is essential for anyone studying optics, including those preparing for exams like the Delhi Police Constable test.