Class 8 Science Notes Chapter 16 (Chapter 16) – Examplar Problems Book
Alright class, let's get straight into Chapter 16 - Light, from your Science Exemplar book. This is a crucial chapter, not just for your class exams but also for various government exams where basic science concepts are tested. Pay close attention!
Chapter 16: Light - Detailed Notes for Exam Preparation
1. What Makes Things Visible?
- We see objects because light from an object enters our eyes.
- This light can either be emitted by the object itself or reflected by the object.
- Luminous Objects: Objects that emit their own light (e.g., Sun, stars, electric bulb, candle flame).
- Non-Luminous Objects (Illuminated Objects): Objects that do not emit their own light but become visible when light falls on them and is reflected towards our eyes (e.g., Moon, planets, chair, book).
2. Laws of Reflection
Reflection is the bouncing back of light after striking a surface.
- Incident Ray: The ray of light that falls on the reflecting surface.
- Reflected Ray: The ray of light that bounces back from the surface after reflection.
- Normal: An imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the incident ray strikes (Point of Incidence).
- Angle of Incidence (∠i): The angle between the incident ray and the normal.
- Angle of Reflection (∠r): The angle between the reflected ray and the normal.
The Two Laws of Reflection:
* First Law: The angle of incidence is always equal to the angle of reflection (∠i = ∠r).
* Second Law: The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.
3. Types of Reflection
- Regular Reflection:
- Occurs when parallel incident rays fall on a smooth, polished surface (like a plane mirror, still water).
- All reflected rays are parallel to each other.
- Forms clear images.
- Diffused or Irregular Reflection:
- Occurs when parallel incident rays fall on a rough or uneven surface (like cardboard, wall, wood).
- The laws of reflection are still obeyed for each individual ray, but due to the uneven surface, the reflected rays scatter in different directions.
- Does not form clear images, but allows us to see the object itself. Most objects around us are visible due to diffused reflection.
4. Image Formation by a Plane Mirror
- Virtual Image: An image that cannot be obtained on a screen (appears to be behind the mirror).
- Erect Image: The image is upright, the same way up as the object.
- Same Size: The image is the same size as the object.
- Laterally Inverted: The left side of the object appears as the right side of the image, and vice-versa (e.g., the word AMBULANCE written in reverse on vehicles).
- Same Distance: The image is formed as far behind the mirror as the object is in front of it.
5. Multiple Images
- When two plane mirrors are placed at an angle to each other, multiple images of an object placed between them are formed due to repeated reflections.
- The number of images depends on the angle (θ) between the mirrors. The smaller the angle, the more images are formed.
- If mirrors are parallel (θ = 0°), an infinite number of images are theoretically formed.
- Kaleidoscope: Works on the principle of multiple reflections by mirrors inclined to each other, creating beautiful patterns.
6. Dispersion of Light
- Sunlight, which appears white, is actually composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
- Dispersion: The phenomenon of splitting white light into its constituent colours when it passes through a transparent medium like a glass prism.
- Spectrum: The band of seven colours obtained after dispersion.
- Rainbow: A natural phenomenon showing dispersion. It occurs when sunlight passes through tiny water droplets suspended in the atmosphere after rain. The droplets act like small prisms.
7. The Human Eye
A complex optical instrument that enables vision.
- Cornea: Transparent outer layer at the front of the eye; refracts (bends) most of the light entering the eye.
- Iris: Coloured, muscular diaphragm behind the cornea; controls the amount of light entering the eye by adjusting the pupil size.
- Pupil: The small opening in the centre of the iris; appears black because light entering it is absorbed inside. Gets smaller in bright light, larger in dim light.
- Lens: A transparent, convex lens located behind the pupil and iris; focuses light onto the retina. Can change its shape (accommodation) to focus on objects at different distances.
- Retina: Light-sensitive screen at the back of the eye; contains photoreceptor cells (rods and cones). Rods are sensitive to dim light, cones are sensitive to bright light and colours. An inverted, real image is formed on the retina.
- Optic Nerve: Transmits electrical signals generated by the rods and cones on the retina to the brain.
- Brain: Interprets the signals, corrects the inversion, and allows us to perceive the object as it is.
- Blind Spot: The point where the optic nerve leaves the retina; there are no photoreceptor cells here, so no vision is possible at this spot.
- Persistence of Vision: The impression of an image stays on the retina for about 1/16th of a second even after the object is removed. This is why we see movies (a series of still pictures shown rapidly) as continuous motion.
8. Care of the Eyes
- Read in proper light (not too dim, not too bright).
- Maintain a suitable reading distance (about 25 cm).
- Wash eyes frequently with clean water.
- Avoid looking directly at the sun or other powerful light sources.
- Do not rub your eyes if something enters; wash with clean water. If irritation persists, see a doctor.
- Include Vitamin A-rich foods in your diet (e.g., carrots, green leafy vegetables, cod liver oil) for good eye health. Deficiency causes night blindness.
9. Visually Challenged Persons and Braille System
- Some people have limited vision or are completely unable to see due to illness, injury, or from birth.
- They develop their other senses (touch, hearing) more sharply.
- Aids for Visually Challenged:
- Non-optical aids: Include tactile aids (Braille writer, slate and stylus), auditory aids (cassettes, talking calculators/computers), electronic aids.
- Optical aids: Include bifocals, contact lenses, tinted lenses, magnifiers, telescopic aids (for suitable candidates).
- Braille System:
- Developed by Louis Braille (who was himself visually challenged).
- Most popular resource for visually challenged persons to read and write.
- Uses raised dots on paper, arranged in patterns called cells.
- Each cell consists of 6 dot positions (2 columns of 3 dots).
- There are 63 dot patterns representing letters, common words, grammatical signs, etc.
- Read by touching the dots.
Multiple Choice Questions (MCQs)
-
According to the law of reflection, which statement is always true?
a) The angle of incidence is greater than the angle of reflection.
b) The angle of incidence is less than the angle of reflection.
c) The angle of incidence is equal to the angle of reflection.
d) The sum of angles of incidence and reflection is 90°. -
Which part of the human eye controls the amount of light entering it?
a) Retina
b) Cornea
c) Iris
d) Lens -
The phenomenon of splitting white light into its constituent colours is called:
a) Reflection
b) Refraction
c) Dispersion
d) Diffraction -
An image formed by a plane mirror is:
a) Real and erect
b) Virtual and inverted
c) Real and inverted
d) Virtual and erect -
Diffused reflection occurs from which type of surface?
a) Smooth and polished surface
b) Rough or irregular surface
c) Transparent surface
d) All surfaces -
The Braille system uses patterns of raised dots. How many dot positions are there in a single Braille cell?
a) 4
b) 6
c) 8
d) 10 -
The impression of an image persists on the retina for about:
a) 1/10th of a second
b) 1/16th of a second
c) 1 second
d) 1/25th of a second -
Which of the following is a luminous object?
a) Moon
b) Planet Jupiter
c) A burning candle
d) A mirror -
In the human eye, the image of an object is formed on the:
a) Pupil
b) Iris
c) Cornea
d) Retina -
A kaleidoscope works on the principle of:
a) Dispersion of light
b) Refraction of light
c) Multiple reflections
d) Persistence of vision
Answer Key for MCQs:
- c) The angle of incidence is equal to the angle of reflection.
- c) Iris
- c) Dispersion
- d) Virtual and erect
- b) Rough or irregular surface
- b) 6
- b) 1/16th of a second
- c) A burning candle
- d) Retina
- c) Multiple reflections
Study these notes thoroughly. Understanding the concepts of light, reflection, and the human eye is fundamental. Remember to relate these concepts to everyday phenomena. Good luck with your preparation!