Light Chapter Notes for CBSE Class 8 Science

Key Terms for Chapter Light

Reflection of Light

The phenomenon of light bouncing back after striking a surface.

Angle of Incidence (∠i)

The angle between the incident ray and the normal at the point of incidence.

Angle of Reflection (∠r)

The angle between the reflected ray and the normal at the point of incidence.

Lateral Inversion

The phenomenon where the right side of an object appears as the left side in its image formed by a plane mirror, and vice versa.

Dispersion

The splitting of white light into its constituent seven colours (VIBGYOR) when it passes through a prism. A rainbow is a natural example.

Retina

The light-sensitive layer at the back of the eye where images are formed. It contains nerve cells called rods and cones.

Blind Spot

The point at the junction of the optic nerve and the retina where there are no sensory cells (rods or cones), so no vision is possible at that spot.

Persistence of Vision

The phenomenon where an image's impression persists on the retina for about 1/16th of a second. This is why we see a continuous picture in movies.

Braille System

A tactile writing system used by visually impaired people, which uses patterns of raised dots to represent letters and numbers.

The Laws of Reflection

Reflection is what allows us to see objects that do not produce their own light. When a ray of light strikes a surface, it bounces off. This bouncing of light is governed by two fundamental laws known as the Laws of Reflection. To understand these laws, we must first define three key components: the incident ray (the light ray that strikes the surface), the reflected ray (the light ray that bounces off the surface), and the normal (an imaginary line drawn perpendicular, i.e., at 90°, to the surface at the point where the incident ray hits).

First Law of Reflection: This law states that the angle of incidence (∠i) is always equal to the angle of reflection (∠r). The angle of incidence is the angle between the incident ray and the normal, while the angle of reflection is the angle between the reflected ray and the normal. So, if light hits a mirror at an angle of 30° to the normal, it will reflect at an angle of 30° to the normal.

Second Law of Reflection: This law states that the incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane. Imagine these three lines drawn on a flat sheet of paper; the reflected ray will not suddenly jump out of the paper. These laws are universal and apply to all types of reflecting surfaces, whether they are smooth like a mirror or rough like a wall.

Regular vs. Diffused Reflection

Must Remember for Exams

• An image formed by a plane mirror is always virtual, erect (upright), of the same size as the object, and laterally inverted.
• The distance of the object from a plane mirror is equal to the distance of the image from the mirror.
• A kaleidoscope works on the principle of multiple reflections from three plane mirrors inclined at 60° to each other.
• Sunlight, or white light, is composed of seven colours (VIBGYOR). The splitting of light into these colours is called dispersion.
• The human eye has a lens that focuses light onto the retina. The retina contains two types of cells: rods (sensitive to dim light) and cones (sensitive to bright light and colour).
• The amount of light entering the eye is controlled by the pupil, whose size is adjusted by the iris.
• The sensation of an image stays on the retina for about 1/16th of a second. This is called persistence of vision.
• The Braille system, developed by Louis Braille, uses 63 dot patterns to represent characters for visually challenged people.

Understanding the Human Eye

The human eye is one of our most important sense organs, functioning like a camera. Light from an object enters the eye through a transparent outer layer called the cornea. The cornea refracts (bends) the light. Behind the cornea is a coloured muscular diaphragm called the iris, which gives the eye its colour. The iris controls the size of a small opening in its centre called the pupil. In bright light, the iris contracts the pupil to limit the amount of light entering, and in dim light, it dilates the pupil to allow more light in.

Behind the pupil is the eye lens, which is a convex lens. The lens focuses the light rays to form a real, inverted, and diminished image on the light-sensitive screen at the back of the eye called the retina. The retina is packed with millions of light-sensitive nerve cells called rods and cones. Cones are responsible for detecting colour and work best in bright light, while rods are responsible for vision in low light conditions (black and white vision). These cells convert the light energy into electrical signals, which are then transmitted to the brain via the optic nerve. The brain interprets these signals and flips the inverted image upright, allowing us to perceive the object as it is.

Exam Preparation Tips

In exams, questions on drawing and labelling diagrams are very common. Practice drawing the ray diagram for reflection and the structure of the human eye multiple times. Ensure all labels (Cornea, Iris, Pupil, Lens, Retina, Optic Nerve) are correct and clearly marked. Remember, the First Law of Reflection (∠i = ∠r) is often tested in numerical problems. If the angle between the incident ray and the mirror is given, you must first calculate the angle of incidence by subtracting it from 90°.

Practice Questions with Solutions

• If the angle between the incident ray and the plane mirror is 40°, what is the angle of reflection? The angle of incidence is 90° - 40° = 50°. According to the law of reflection, the angle of reflection is equal to the angle of incidence. So, the angle of reflection is 50°.
• What kind of image is formed by a plane mirror? A plane mirror forms a virtual, erect, and laterally inverted image of the same size as the object.
• Which part of the eye contains light-sensitive cells called rods and cones? The retina contains rods and cones.
• What is the principle behind the working of a kaleidoscope? A kaleidoscope works on the principle of multiple reflections from inclined mirrors.

Frequently Asked Questions on Light

Frequently Asked Questions