CBSE Notes Class 8 Science Chapter 13 Light

Light is a form of energy that enables us to perceive the world around us. It travels in straight lines, and understanding its properties helps us comprehend various scientific concepts and everyday phenomena. Light is crucial in numerous technologies, including cameras, telescopes, and fiber optics. Additionally, it plays an important role in processes like photosynthesis in plants, making it essential for life on Earth.

1.0Reflection of Light

The process of light bouncing back when it encounters a boundary between different media that it cannot penetrate is known as reflection.

2.0Laws of Reflection

1. Ray of Light: : A ray of light is a hypothetical concept representing a single, thin line of light. In reality, many rays combine to form a beam of light.
2. Incident Ray: The light ray that hits any surface is known as the incident ray.
3. Reflected Ray: The ray that returns from the surface after reflection is referred to as the reflected ray.
4. Normal: A line drawn perpendicular to the surface of the mirror at the point where the incident ray hits it.
5. Angle of incidence: It is the angle created between the normal and the incident ray, commonly represented as i
6. Angle of reflection: The angle of reflection is defined as the angle between the normal and the reflected ray, commonly represented as r.

Laws of Reflection

Law 1 : The incident ray, the normal  and the reflected ray all lie in the same plane.

Law 2 : Angle of incidence is equal to the angle of reflection

Law 3 or Law of Lateral Inversion: In a mirror, the left side of an object appears on the right, and the right side appears on the left. This effect is called lateral inversion.

3.0Regular And Diffused Reflection

• Regular Reflection: When light drops on a smooth uniform surface (like a mirror), all the parallel incident rays are reflected perfectly parallel. This is called Regular Reflection.
• Diffused Reflection: When light strikes a rough surface, such as stone, skin, or clothing, the parallel incident rays scatter in various directions instead of reflecting parallel to each other. This phenomenon is known as diffused reflection.

Note: Both Regular and Diffused Reflection obey the Laws of Reflection. Diffused reflection is caused by irregularities in the reflecting surface and not due to the failure of Laws of Reflection.

4.0Luminous and Non-Luminous objects

• Luminous : Objects which emit light of their own. Eg: Sun, Bulb, Candle, Fire.
• Illuminated or Non-luminous :Objects that do not emit their own light but instead reflect light from luminous sources, making them visible to our eyes. Examples include the moon, blue sky, clouds, trees, cars, and tables.

5.0Components of White Light

• Sunlight or white light is made of seven colours as in VIBGYOR(Violet, Indigo, Blue, Green, Yellow, Orange, Red).
• The process of separating white light into its constituent colors as it passes through a medium, like a prism, is known as dispersion.

6.0Human Eye

• Light mainly tumbles on the cornea - the white clear front part of the eye. It is tough and guards the interior of the eye from accidents.
• The iris (a muscular structure behind the cornea) of the eye controls the amount of light reaching the interior of the eye by automatically adjusting the size of the pupil. (The iris is that part of the eye which gives it the colour such as black, brown, green eyes commonly called cat-eyes etc)
• The eye's crystalline biconvex shaped lens is located behind the pupil. It further focuses the light on the retina. The retina contains several nerve cells which convert the optical images to signals to the brain via the optic nerve.
• There are 2 types of nerve cells. 
• (i) Cones : which are responsive to bright light and help us recognize color 
• (ii) Rods: which are responsive to dim light.
• The brain then decodes this signal as the image we see

Note : The light is focused by the cornea, then the iris by controlling the opening of the pupil and then the lens. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.

Care of Eyes:

• Eyes are sensitive and essential, so it’s important to take good care of them. If you experience pain, irritation, or poor visibility, consult a doctor promptly. Use appropriate spectacles or lenses if recommended.
• To avoid eye strain, ensure you have sufficient lighting for daily activities. Reading should be done at a distance of about 25 cm, and the same applies when watching TV or using devices like laptops and tablets. Avoid rubbing your eyes too hard and wash them regularly with clean water. If irritation continues, seek medical advice.
• A healthy diet contains Vitamin A—present in foods like carrots, leafy greens, eggs, milk, and dairy products—can aid in preventing eye problems.

7.0Braille Script

• Some individuals may experience visual impairment due to congenital conditions or loss of sight from accidents or diseases. They rely on their other senses—such as touch, hearing, and speech—to communicate and engage with the world around them.
• They use aids like non-optical devices (hearing aids, Braille kits, audio materials) and optical devices (lenses, bifocals). Braille is a widely used method for reading among the visually impaired.
• The Braille script is an alphabet system designed for the visually impaired to read and write. Each Braille character consists of a combination of six dots arranged in a matrix. These dots can be raised or flat, and individuals who are blind can touch and feel the patterns of raised dots to recognize the letters.

8.0Sample Question on Light

Q. Create a clear and well-labeled diagram of the human eye.

Solution: