Notes For All Chapters Science Class 7
1. Light Travels Along a Straight Line
- Light can be seen as a beam when it enters a room through a small hole or narrow opening.
- Beams of light are seen from torches, vehicle headlights, searchlights, and lighthouses.
- An experiment with a straight pipe and a bent pipe shows that:
- A candle flame is visible through a straight pipe.
- It is not visible through a bent pipe.
- This proves that light travels in a straight line.
2. Reflection of Light
When light falls on a shiny or polished surface, it changes its direction.
This change in direction of light is called reflection of light.
Surfaces like:
- Mirrors
- Shiny steel plates
- Water surface
can act as mirrors.
Plane Mirror
- A flat mirror is called a plane mirror.
- When light falls on a plane mirror, it gets reflected.
3. Image Formation by a Plane Mirror
Object and Image
- The object is the actual thing (for example, a candle).
- The image is what we see in the mirror.
Characteristics of Image Formed by a Plane Mirror
The image is:
- Erect (upright)
- Virtual (cannot be obtained on a screen)
- Same size as the object
- Formed behind the mirror
The distance of the image behind the mirror is equal to the distance of the object in front of the mirror.
4. Real Image and Virtual Image
Real Image
- Can be obtained on a screen.
- Example: Image formed by a concave mirror on a paper screen.
Virtual Image
- Cannot be obtained on a screen.
- Example: Image formed by a plane mirror.
5. Lateral Inversion (Right or Left)
In a plane mirror:
- The right side appears left
- The left side appears right
- This is called lateral inversion.
- The image is not upside down; only sides are interchanged.
Example:
The word AMBULANCE is written in reverse on the front of the vehicle so that it appears correct in rear-view mirrors.
6. Spherical Mirrors
- Mirrors with curved reflecting surfaces are called spherical mirrors.
- They are parts of a sphere.
Types of Spherical Mirrors
(a) Concave Mirror
Reflecting surface curves inward.
Can form:
- Real and inverted images
- Virtual, erect, and magnified images when the object is very close.
Uses:
- Doctors and dentists use them to see enlarged images.
- Reflectors of torches and vehicle headlights.
(b) Convex Mirror
Reflecting surface bulges outward.
Always forms:
- Virtual
- Erect
- Smaller images
Uses:
Side-view mirrors in vehicles because they cover a large area.
7. Images Formed by Concave and Convex Mirrors
Concave Mirror
Image can be:
- Smaller or larger than the object
- Real or virtual
Real images can be obtained on a screen.
Convex Mirror
Image is:
- Always virtual
- Always erect
- Always smaller
Real image cannot be obtained.
8. Lenses
A lens is a transparent object through which light can pass.
Lenses are used in:
- Spectacles
- Microscopes
- Telescopes
- Magnifying glass
Types of Lenses
(a) Convex Lens
- Thicker in the middle than at the edges.
- Converges light rays, so it is called a converging lens.
Can form:
- Real and inverted images
- Virtual, erect, and magnified image when the object is very close
Used as a magnifying glass.
(b) Concave Lens
- Thinner in the middle than at the edges.
- Diverges light rays, so it is called a diverging lens.
Always forms:
- Virtual
- Erect
- Smaller images
9. Sunlight – White or Coloured?
A rainbow is seen after rain when the Sun is low in the sky.
It has seven colours:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
Sunlight is white light.
White light is a mixture of seven colours.
Prism Experiment
- A glass prism splits sunlight into seven colours.
- This shows that sunlight is made up of seven colours.
10. Newton’s Disc
- A disc painted with seven rainbow colours.
- When rotated fast, the colours mix and the disc appears white.
- This proves that white light is a mixture of seven colours.
Good