Light — NCERT Solutions, AI Tutor & Practice

Understanding reflection: When light strikes a smooth surface (like a plane mirror), it bounces back. This bouncing is called reflection.

The two laws of reflection are:

1. First law: The angle of incidence ($\angle i$) is always equal to the angle of reflection ($\angle r$).

$\angle i = \angle r$

1. Second law: The incident ray, the reflected ray, and the normal (the line drawn perpendicular to the mirror at the point of incidence) all lie in the same plane.

Note: The angles of incidence and reflection are always measured from the normal, not from the mirror surface.

Step 1 — Find the angle of incidence. The angle of incidence is measured from the normal, not the mirror. The ray makes $30^\circ$ with the mirror surface, so with the normal it makes:

$\angle i = 90^\circ - 30^\circ = 60^\circ$

Step 2 — Apply the law of reflection. By the first law of reflection:

$\angle r = \angle i = 60^\circ$

Answer: The angle of reflection is $60^\circ$.

Regular Reflection:

• Occurs on a smooth, polished surface (e.g., a plane mirror, still water).
• Parallel incident rays remain parallel after reflection.
• Produces a clear image.

Diffused (Irregular) Reflection:

• Occurs on a rough or uneven surface (e.g., paper, wall, road).
• Parallel incident rays are scattered in different directions.
• Does not produce a clear image.

Important point: In both cases, the laws of reflection are obeyed at each point. Diffused reflection happens because the normals at different points of a rough surface point in different directions.

The image formed by a plane mirror has the following characteristics:

1. Virtual — the image cannot be caught on a screen (it appears to be behind the mirror).

2. Erect — the image is upright, the same way up as the object.

3. Same size — the image is exactly the same size as the object.

4. Equal distance — the image is formed at the same distance behind the mirror as the object is in front of it.

5. Laterally inverted — the left side of the object appears as the right side of the image (and vice versa). This is called lateral inversion.

Definition: Lateral inversion is the phenomenon in which the left side of an object appears as the right side of its image, and the right side appears as the left, when seen in a plane mirror.

Everyday example: The word AMBULANCE is written in a reversed (mirror-image) form on the front of ambulance vehicles:

$\text{ECNALUBMA}$

When a driver ahead looks into their rear-view mirror, the reversed word appears correct as AMBULANCE, so they can quickly read it and give way.

Step 1 — Recall the formula. The number of images $n$ formed by two plane mirrors inclined at an angle $\theta$ is:

$n = \frac{360^\circ}{\theta} - 1$

Step 2 — Substitute $\theta = 60^\circ$.

$n = \frac{360^\circ}{60^\circ} - 1 = 6 - 1 = 5$

Answer: 5 images will be formed.

This is exactly the principle used in a kaleidoscope, where mirrors placed at an angle create many beautiful images.

Main parts of the human eye:

1. Cornea — the transparent front part that lets light enter.
2. Iris — the coloured part that controls the size of the pupil.
3. Pupil — the opening through which light enters; it adjusts in size.
4. Lens — focuses light onto the retina.
5. Retina — the light-sensitive screen at the back of the eye.
6. Optic nerve — carries signals to the brain.

Function of the retina: The retina is the screen on which a real and inverted image is formed. It contains light-sensitive cells:

• Cones — sensitive to bright light and colour.
• Rods — sensitive to dim light.

These cells convert light into signals sent through the optic nerve to the brain, which interprets the image as upright.

Definition: The least distance of distinct vision is the minimum distance at which an object can be seen most clearly and comfortably without straining the eyes.

For a normal human eye (adult), this distance is:

$25\ \text{cm}$

If an object is brought closer than this, the eye cannot focus it clearly and it appears blurred, causing strain.

Working of a periscope:

A periscope uses two plane mirrors fixed inside a long tube, each set at 45° to the walls of the tube.

Steps:

1. Light from a distant object first strikes the top mirror (tilted at 45°).
2. This mirror reflects the light downward by turning it through 90° (using the law of reflection).
3. The light travels down the tube and hits the bottom mirror (also at 45°).
4. The bottom mirror reflects the light towards the observer's eye, again turning it 90°.

Result: The observer can see objects that are otherwise hidden — for example, over a high wall or the surface of the sea from inside a submarine.

The reflection follows the law $\angle i = \angle r$ at both mirrors.

Understanding the need: Visually impaired (blind) persons cannot use their sense of sight, so they read using their sense of touch.

What is Braille? Braille is a special writing system developed by Louis Braille. It uses patterns of raised dots arranged in cells.

How it works:

• Each Braille cell has up to six raised dots (arranged in two columns of three).
• Different arrangements of these raised dots represent different letters, numbers, and symbols.
• A person moves their fingertips over the dots and, by feeling their pattern, recognises the characters.

Conclusion: Braille allows visually impaired people to read books and write, giving them access to education and information through the sense of touch.

Step 1 — Recall the property. In a plane mirror, the image is formed at the same distance behind the mirror as the object is in front of it.

Step 2 — New object distance. Original object distance $= 15$ cm. Moved closer by $5$ cm, so new object distance:

$15\ \text{cm} - 5\ \text{cm} = 10\ \text{cm}$

Step 3 — Image distance behind mirror. The image is also $10$ cm behind the mirror.

Step 4 — Distance between object and image.

$10\ \text{cm} + 10\ \text{cm} = 20\ \text{cm}$

Answer: The object and its image are now 20 cm apart.

Precautions to keep the eyes healthy:

1. Read in proper light — neither too dim nor too bright, to avoid eye strain. Keep the book at about $25$ cm (the least distance of distinct vision).

2. Do not look directly at the Sun or other strong light sources, as they can damage the retina.

3. Eat a balanced diet rich in Vitamin A (found in carrots, papaya, green vegetables, milk) to prevent night blindness.

4. Wash your eyes with clean water regularly and avoid rubbing them with dirty hands.

Additional care: Get eyes checked regularly; use spectacles if advised; avoid too much screen time and give eyes rest.