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Chapter 16. Light

• Light travels in a straight line.

• Light does not need a medium (can travel through vacuum).

• The speed of light in air ≈ 3 × 10⁸ m/s. 

• Reflection: Bouncing back of light from a surface.

• Laws of Reflection:

  1. Angle of incidence = Angle of reflection.

  2. Incident ray, reflected ray and normal lie in the same plane.

1. Regular Reflection: Smooth surface (mirror), forms clear image.

2. Diffuse Reflection: Rough surface, light scattered, no clear image.

• Plane Mirror:

  • Image is virtual, erect, same size.

  • Image is laterally inverted (left ↔ right).

• Multiple Reflection: When two mirrors are placed at an angle, multiple images are formed (used in kaleidoscope, periscope).

• Eye is like a camera, forms image on retina.

• Parts:

  • Cornea: Transparent front covering.

  • Iris: Coloured part, controls size of pupil.

  • Pupil: Controls light entry.

  • Lens: Converging lens, focuses light on retina.

  • Retina: Light-sensitive layer.

  • Optic nerve: Sends signals to brain.

• Blind spot: Point on retina where no vision is possible.

• Persistence of vision: Image stays on retina for 1/16th second.

1. Myopia (Short-sightedness): Cannot see distant objects → corrected by concave lens.

2. Hypermetropia (Long-sightedness): Cannot see nearby objects → corrected by convex lens.

• Proper lighting while reading.

• Balanced diet (Vitamin A prevents night blindness).

• Avoid watching screens for long time.

• Script for visually challenged people.

• Uses raised dots (patterns) to represent letters and words.

• Invented by Louis Braille.

Chapter 16. Light

Chapter 16: Light (Class 8 Science)

NCERT In-text & Exercise Questions with Answers

Answer:

• In a dark room, objects are not visible because no light is reflected from them to our eyes.

• If there is light outside and the door/window is open, then we can see objects outside the room because light from them enters our eyes.

Answer:

• Regular Reflection: Occurs on a smooth surface (like a plane mirror). Reflected rays are parallel, and a clear image is formed.

• Diffused Reflection: Occurs on a rough surface. Reflected rays are scattered in different directions, no clear image is formed.

• Important: Diffused reflection does not mean the laws of reflection fail. Each individual ray still obeys the laws of reflection; only the reflected rays go in different directions.

(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper

Answer:
(a) Polished wooden table – Regular reflection (smooth surface)
(b) Chalk powder – Diffused reflection (rough surface)
(c) Cardboard surface – Diffused reflection (uneven surface)
(d) Marble floor with water spread over it – Regular reflection (smooth with water)
(e) Mirror – Regular reflection (very smooth surface)
(f) Piece of paper – Diffused reflection (rough under microscope, though seems smooth to us)

Answer:

1. The angle of incidence = The angle of reflection.

2. The incident ray, the reflected ray, and the normal to the reflecting surface all lie in the same plane.

Answer:

• Place a plane mirror vertically on a table (mirror base at right angle).

• Fix a white sheet of paper on the table under the mirror.

• Draw a normal and incident ray on the paper, and mark the reflected ray by using pins.

• If we fold/tilt the paper, the reflected ray is not visible on it — this proves all three (incident ray, reflected ray, and normal) lie in the same plane.

(a) The angle of incidence is equal to the __________.
(b) The incident ray, the normal and the reflected ray all lie in the same __________.
(c) Image formed by a plane mirror is __________ and __________.

Answer:
(a) Angle of reflection
(b) Plane
(c) Virtual and erect

Answer:
 In the exam, draw a neat diagram with labels:

• Cornea, Pupil, Iris, Lens, Retina, Optic Nerve, Ciliary muscles, Aqueous humor, Blind spot, etc.

Answer:

• Read in proper light (not very dim or very bright).

• Keep safe distance from TV, computer, and mobile screens.

• Eat a balanced diet rich in Vitamin A (carrots, spinach, milk, fish, etc.).

• Avoid reading in moving vehicles.

• Have regular eye checkups.

Answer:
Angle between incident ray and reflected ray = 90°.
So, ∠i + ∠r = 90°
But ∠i = ∠r
So, 2∠i = 90° → ∠i = 45°

Answer: Angle of incidence = 45°.

Answer:
An infinite number of images will be formed (because the mirrors keep reflecting the images of each other endlessly).

Answer (explained):

• At the first mirror: Angle of incidence = 30°, so angle of reflection = 30°.

• At the second mirror: Incident ray falls at 60° (complementary to 30°), so it reflects at 60°.

• The path can be drawn showing two reflections.
  ✅ The law of reflection holds at each mirror.

Answer:

• He cannot see himself because his image is not formed in the mirror at that position.

• He can see objects at Q and R, because their rays can reflect into his eyes.

• He cannot see object at P (outside reflected region).

Choose the correct option:
(a) The size of the pupil is controlled by – (i) Iris (ii) Cornea (iii) Lens (iv) Retina
(b) The eye lens becomes thin when – (i) Ciliary muscles relax (ii) Ciliary muscles contract (iii) Pupil expands (iv) Iris contracts

Answer:
(a) Iris
(b) Ciliary muscles relax