Science Concept Notes for Class 8: Chapter 16 Light
Explore reliable concept summaries and revision materials through the CBSE Class 8 Science Light Notes with Questions Set 03. Tailored for Class 8 learners, utilizing these notes for Science Chapter 16 Light ensures thorough preparation and strengthens core academic definitions before final evaluations.
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Light and Reflection
Light is a form of energy that gives us the sensation of vision. We see an object when light reflected from it enters our eyes. Light travels in a straight line and forms shadows.
Laws of reflection: (i) The angle of incidence is equal to the angle of reflection. (ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. These laws apply to all reflecting surfaces, including curved ones.
Image in a Plane Mirror
Rays from an object reflect from the mirror and, when extended backwards, appear to meet behind it; the image is formed there. The image is always behind the mirror, virtual (cannot be taken on a screen), erect, laterally inverted and the same size as the object. It is as far behind the mirror as the object is in front.
Regular and Diffused Reflection
| Type | Surface | What happens | Example |
|---|---|---|---|
| Regular reflection | Smooth and polished | Parallel rays are reflected parallel to each other in one direction | Plane mirror, still water |
| Diffused (irregular) reflection | Rough | Parallel rays are reflected in different directions | Wall of a room, a tree, paper |
Multiple Images and the Kaleidoscope
When two mirrors are placed at an angle, an object between them gives several images, because the image in one mirror acts as an object for the other. The number of images is n = (360° / angle between the mirrors) − 1. At 90° we get 3 images, at 60° we get 5 images, and with two parallel facing mirrors we get an infinite number of images (as in a barber's shop). A kaleidoscope uses this idea to make beautiful patterns: three mirror strips are joined into a prism and fitted in a tube with coloured glass pieces or beads at one end.
The Human Eye
| Part | Function |
|---|---|
| Cornea | Transparent front window; bends most of the incoming light |
| Iris | Coloured ring of muscle that controls the amount of light entering the eye |
| Pupil | Adjustable opening in the iris through which light enters. It becomes smaller in bright light and larger in dim light |
| Lens | Transparent convex lens whose thickness is changed by the ciliary muscles to focus objects at different distances |
| Retina | Light-sensitive screen at the back of the eye where the image forms; it holds cone cells (bright light and colour) and rod cells (dim light) |
| Optic nerve | Carries signals from the retina to the brain |
| Blind spot | Point where the optic nerve leaves the retina; it has no sensory cells, so no image is seen there |
Working: light from an object passes through the cornea, pupil and lens and forms a real, small, inverted image on the retina. The retina changes it into electrical signals, which the optic nerve carries to the brain, and the brain interprets it as an upright picture.
Persistence of vision: an image stays on the retina for about 1/16 second. If still pictures are shown faster than 16 per second, they seem to move, which is how movies and animation work.
Care of the Eyes
Visit an eye specialist for any problem. Avoid too bright or too dim light. Never look directly at the Sun or a strong light. Do not rub your eyes; wash them with clean water if dust enters. Read at the normal distance, not too close or too far. Eat food rich in vitamin A, such as carrots, milk and green leafy vegetables.
Defects of Vision
| Defect | Problem | Correction |
|---|---|---|
| Myopia (short sight) | Nearby objects are clear, distant ones are blurred | Concave lens spectacles |
| Hypermetropia (long sight) | Distant objects are clear, nearby ones are not | Convex lens spectacles |
| Cataract | The lens becomes cloudy in old age and vision is dim | Surgery to replace the lens with an artificial one |
Visually Challenged Persons and Braille
People with partial or complete loss of vision usually develop their senses of hearing and touch very well. Optical aids (bifocal lenses, magnifiers and telescopic aids) help the partially sighted. Non-optical aids include tactual aids such as the Braille writer and slate and stylus, auditory aids such as talking books and recorders, and electronic aids such as talking calculators and closed-circuit television that enlarges print.
Braille was developed by Louis Braille, who was born in France in 1809 and lost his sight in an accident as a child. He devised the script in 1824. It has 63 patterns of raised dots arranged in cells of two columns of three dots; each pattern stands for a letter, a letter combination, a common word or a punctuation sign, and is read by touch.
Questions and Answers
1. State the two laws of reflection.
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Answer: The angle of incidence equals the angle of reflection; the incident ray, normal and reflected ray lie in the same plane.
Teacher's Note: Use the key term in your answer.
2. Name the features of the image in a plane mirror.
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Answer: Virtual, erect, laterally inverted, same size as the object, and as far behind the mirror as the object is in front.
Teacher's Note: Give an example if the question allows.
3. How many images form when two mirrors are at 60° to each other?
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Answer: n = 360/60 − 1 = 5 images.
Teacher's Note: Write a short, clear answer.
4. Which part of the eye controls the amount of light entering it?
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Answer: The iris, by changing the size of the pupil.
Teacher's Note: Remember this for your exam.
5. What is the blind spot?
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Answer: The point where the optic nerve meets the retina; it has no light-sensitive cells.
Teacher's Note: Learn this point and write it in your own words.
6. What is persistence of vision?
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Answer: The impression of an image stays on the retina for about 1/16 of a second.
Teacher's Note: Use the key term in your answer.
7. Which lens corrects myopia and which corrects hypermetropia?
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Answer: A concave lens corrects myopia and a convex lens corrects hypermetropia.
Teacher's Note: Give an example if the question allows.
8. Who developed the Braille system and how many characters does it have?
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Answer: Louis Braille; it has 63 dot patterns.
Teacher's Note: Write a short, clear answer.
9. Why does a wall give diffused reflection?
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Answer: Its surface is rough, so parallel rays are reflected in many different directions.
Teacher's Note: Remember this for your exam.
Free study material for Science
Download CBSE Revision Notes: Class 8 Science Chapter 16 Light
Quick Revision Notes: Chapter 16 Light (CBSE)
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