In this post, you will learn about Reflection of Light in Class 12 Physics, including laws, mirror formulas, ray diagrams, and real-life applications important for CBSE, RBSE, NEET, and JEE exams.
What is the reflection of light in Physics?
Definition of Reflection of Light
Reflection of light is the phenomenon in which light rays bounce back into the same medium after striking a surface.
Key Concepts of Reflection of Light (Class 12 Physics)
Laws of Reflection:
First Law of Reflection:
Angle of incidence is equal to angle of reflection.
This law is valid for all reflecting surfaces and ensures that the path of light is predictable.
Important Points
- Angle of incidence (i) = Angle of reflection (r): The angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal.
- Measurement: Angles are measured with respect to the normal, which is an imaginary line drawn perpendicular to the reflecting surface at the point where the light ray strikes.
Second Law of Reflection (Coplanar Rays):
Incident ray, reflected ray, and normal lie in the same plane.
Regular Reflection (Specular)
Occurs on smooth surfaces (like mirrors), producing clear, crisp images.
Diffuse Reflection (Irregular)
Occurs on rough surfaces, scattering light in various directions.
Important Terms in Reflection of Light
Incident Ray
The incoming light ray that approaches and strikes a surface is known as the incident ray.
Reflected Ray
The light ray that leaves the surface after interaction and returns into the same medium is called the reflected ray.
Normal
A hypothetical line drawn at a right angle to the surface at the point of contact is called the normal.
Point of incidence
The exact location on the surface where the incoming ray makes contact is termed the point of incidence.
Reflecting Surface
A surface capable of sending light back after it strikes it is known as a reflecting surface.
Reflection by Spherical Mirrors
Spherical mirrors, including concave (inward-curving) and convex (outward-curving) types, reflect light according to the laws of reflection. Concave mirrors converge light, producing real/virtual and inverted/erect images, while convex mirrors diverge light, creating virtual, erect, and diminished images.
Types of Spherical Mirrors
Concave Mirror
Reflects light from its inner surface and converges parallel rays to a focal point in front of the mirror. Often used for magnification (e.g., shaving mirrors, solar furnaces).
Convex Mirror
Reflects light from its outer surface and diverges parallel rays, making them appear to originate from a virtual focus behind the mirror. Used for, e.g., rear-view mirrors, as they provide a wider field of view.
Key Terms
Lateral Inversion: Lateral inversion is the apparent reversal of left and right in an image formed by a mirror, where the left side of the object appears as the right side in the image and vice versa.
Ray Diagram Rules for Mirrors
To draw ray diagrams for spherical mirrors, the following standard rules are used:
1. A ray parallel to the principal axis, after reflection, passes through the focus (in case of a concave mirror) or appears to diverge from the focus (in case of a convex mirror).
2. A ray passing through the focus of a concave mirror, after reflection, becomes parallel to the principal axis. In a convex mirror, a ray directed towards the focus reflects parallel to the principal axis.
3. A ray passing through the centre of curvature reflects back along the same path.
4. A ray incident at the pole of the mirror reflects obeying the laws of reflection, making equal angles with the principal axis.
Formula Explanation
`frac{1}{f} = frac{1}{v} + frac{1}{u}`
“This formula helps in calculating image position and focal length of mirrors, but this is valid only for spherical mirrors under the Cartesian sign convention“.
Where:
f = focal length
v = image distance
u = object distance
Sign Convention
To correctly use the mirror formula, we follow the Cartesian sign convention, which provides a standard way to assign signs to distances and heights:
– All measurements are taken from the pole (vertex) of the mirror.
– Any distance measured in the direction of incident light (usually to the right of the mirror) is taken as positive, while those measured in the opposite direction are taken as negative.
– Heights measured above the principal axis are assigned a positive value.
– Heights measured below the principal axis are assigned a negative value.
Real-Life Applications of Reflection of Light in Daily Life
Mirrors in Daily Use
1. Plane mirrors are commonly found in dressing tables, interior decoration, and periscopes.
2. Concave mirrors are preferred for shaving and makeup because they produce enlarged images.
3. Convex mirrors are installed in vehicles as rear-view mirrors due to their wide field of vision.
Reflecting Telescopes
1. These telescopes are used in astronomy to study faraway celestial objects like stars and planets.
2. They rely on concave mirrors to gather and focus incoming light rays.
3. A well-known example is the Hubble Space Telescope.
Solar Cookers & Solar Furnaces
1. Devices such as solar cookers and furnaces use concave mirrors to concentrate sunlight.
2. This focused energy is converted into heat for cooking and industrial purposes.
Periscopes
1. Periscopes are used in submarines and defence applications to view objects not in direct line of sight.
2. They operate on repeated reflection using plane mirrors.
Headlights & Torches
1. In headlights and torches, concave mirrors help convert light into a strong, parallel beam.
2. This enhances visibility, especially at night.
Optical instruments
1. Reflection plays a key role in instruments such as microscopes, cameras, and binoculars.
2. It helps in controlling light and forming clear images.
Safety and Surveillance
1. Convex mirrors are widely used for safety purposes at road intersections and sharp turns, and in stores and parking areas for monitoring.
Kaleidoscope
1. A kaleidoscope uses multiple reflections to create attractive and symmetrical patterns.
Road Reflectors and Signs
1. Special reflectors on road signs bounce back the light from vehicle headlights toward the driver.
2. This makes signs visible even in low-light conditions.
Medical Applications
1. Dentists use concave mirrors to observe enlarged images of teeth for better examination.
Quick Revision
Reflection of light is widely used in mirrors, optical instruments, telescopes, solar devices, safety mirrors, and daily life applications for image formation, light focusing, and visibility.
Conclusion
This topic helps you build a strong foundation in understanding how light behaves when it interacts with surfaces. By learning these concepts, you can confidently solve numerical problems, draw accurate ray diagrams, and avoid common mistakes in exams. This knowledge also connects theory with real-life situations, making the subject more practical, interesting, and easier to remember.
Light Reflection Important Questions with Answers
Question
Define the following terms used in the reflection of light
(a) Incident ray
(b) Normal
(c) Reflected ray
(d) Angle of incidence
(e) Angle of reflection
Question
The power of a plane mirror is –
(a) `infty`
(b) 0
(c) 2 D
(d) 4 D
Answer:
(b) 0
Question
A plane mirror reflects a pencil of light to form a real image. Then the pencil of light incident on the mirror is –
(a) Parallel
(b) Convergent
(c) Divergent
(d) None of the above
Answer:
(b) Convergent
Question
A man runs towards a mirror at a speed `15 frac m/s}` . The speed of the image relative to the man is –
(a) `15 ms^{-1}`
(b) `30 ms^{-1}`
(c) `35 ms^{-1}`
(d) `20 ms^{-1}`
Answer:
(b) `30 ms^{-1}`
Question
A man having height 6 m. He observes image of 2 m height erect, then mirror used is –
(a) Concave
(b) Convex
(c) Plane
(d) None of these
Answer:
(b) Convex
Question
Two mirrors are inclined at an angle of `50^circ`. The number of images formed for an object placed between the mirrors is –
(a) 5
(b) 6
(c) 7
(d) 8
Answer:
(c) 7
Question
A light bulb is placed between two plane mirrors inclined at an angle of `60^circ`. The number of images formed is –
(a) 6
(b) 2
(c) 5
(d) 4
Answer:
(c) 5
Question
Two plane mirrors are inclined to each other such that a ray of light incident on the first mirror and parallel to the second is reflected from the second mirror parallel to the first mirror. The angle between the two mirrors is –
(a) `30 ^circ`
(b) ` 45 ^circ`
(c) ` 60 ^circ`
(d) ` 75 ^circ`
Answer:
(c) ` 60 ^circ`
Question
A thick plane mirror shows a number of image of the filament of an electric bulb. Of these, the brightest image is the –
(a) First
(b) Second
(c) Fourth
(d) Last
Answer:
(b) Second
Question
A man is 180 cm tall and his eyes are 10 cm below the top of his head. In order to see his entire height right from toe to head, he uses a plane mirror kept at a distance of 1 m from him. The minimum length of the plane mirror required is –
(a) 180 cm
(b) 90 cm
(c) 85 cm
(d) 170 cm
Answer:
(b) 90 cm
light Numerical
Q. A man approaches a vertical plane mirror at speed of `2 frac{m}{s}`. At what rate does he approach his image?
Image formed in a plane mirror is at same distance from the mirror.
In this condition object and image are in same speed but in opposite direction.
`vec {v} = vec {v_I} – vec{v_O}`
`vec {v} = – vec {v_O} – vec{v_O}` `{because vec{v_I} = – vec {v_O}}`
`vec {v} = – 2 vec {v_O} `
This is the velocity of the image as observed by the observer
In terms of magnitude
`v = – 2 (2) `
`v = – 2 times 2 `
`v = – 4 frac {m}{s} `
So, the magnitude is `4frac{m}{s}`.