CBSE Class 7 Science Advanced Physics Light Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. An electric bulb converts ________ energy to ________ energy.
Answer:
An electric bulb converts electrical energy to light energy.
In simple words: A bulb makes light from electricity.

Teacher's Note:
1. Some energy also appears as heat.
2. The filament glows when hot.

Question 2. Real images are formed by ________ beam of light.
Answer:
Real images are formed by a convergent beam of light.
In simple words: The rays actually meet.

Teacher's Note:
1. Convergent rays come together at a point.
2. That point can be caught on a screen.

Question 3. Virtual images are formed by ________ beam of light.
Answer:
Virtual images are formed by a divergent beam of light.
In simple words: The rays only seem to meet when extended backwards.

Teacher's Note:
1. Divergent rays spread apart.
2. The eye traces them back to a point.

Question 4. ________ helps to convert light energy into chemical energy.
Answer:
Chlorophyll helps to convert light energy into chemical energy.
In simple words: The green pigment in leaves does the job.

Teacher's Note:
1. This happens in photosynthesis.
2. It also gives leaves their green colour.

Question 5. When the Moon is completely not visible from the Earth, we call it ________ eclipse.
Answer:
We call it a total lunar eclipse.
In simple words: The whole Moon goes dark.

Teacher's Note:
1. If only part of the Moon is hidden, it is a partial lunar eclipse.
2. A lunar eclipse happens only on a full-moon night.

Question 6. A concave mirror is formed from a sphere of radius 20 cm. The image of an object at an infinite distance is formed at a distance of ________ cm from the mirror.
Answer:
For an object at infinity the image forms at the principal focus.
f = R/2 = 20/2 = 10 cm.
In simple words: The image sits at half the radius from the mirror.

Teacher's Note:
1. Use f = R/2.
2. The image is highly diminished and real.

Question 7. ________ mirror is used as shaving mirror.
Answer:
A concave mirror is used as a shaving mirror.
In simple words: It gives a magnified upright image of the face.

Teacher's Note:
1. The face must lie between the pole and the focus.
2. Dentists use the same type of mirror.

Question 8. If the angle of incidence on a plane mirror is 30°, then the angle between the reflected ray and the plane mirror is ________.
Answer:
Angle of reflection = angle of incidence = 30°.
Angle with the mirror = 90° − 30° = 60°.
In simple words: Angles from the normal and from the mirror add up to 90°.

Teacher's Note:
1. Draw the normal to see this.
2. Do not confuse angle with the mirror and angle with the normal.

Question 9. If the object is placed between F and C of a concave mirror, the image is formed at ________.
Answer:
The image is formed beyond C.
In simple words: The image is farther than the centre of curvature.

Teacher's Note:
1. It is real, inverted and magnified.
2. As the object moves from C towards F the image moves farther away.

Teacher's Note:
1. Do the sure ones first.
2. The image on the retina is real and inverted.

Teacher's Note:
1. Homogeneous means the same composition everywhere.
2. Bioluminescence is light made by living things.

Teacher's Note:
1. Rainbow = dispersion of sunlight by raindrops.
2. A filament bulb is hot when it glows.

Teacher's Note:
1. Convex: wide view. Concave: magnified view.
2. A kaleidoscope uses plane mirrors.

Teacher's Note:
1. Inner surface curves inwards (concave).
2. Outer surface bulges out (convex).

Teacher's Note:
1. R = 2f.
2. A tube light is bigger than a point.

Teacher's Note:
1. Give two examples for each.
2. A candle changes from non-luminous to luminous when lit.

Question 17. Define an optical medium.
Answer:
Any material through which light can pass, fully or partly, is an optical medium, such as air, water or glass.
In simple words: A material that light can travel through.

Teacher's Note:
1. Media can be transparent, translucent or opaque.
2. Vacuum also lets light pass.

Question 18. What are point source and extended source of light?
Answer:
A point source is a source as small as a pin head. An extended source is larger than that, for example a tube light or a burning candle.
In simple words: Small dot-like source versus bigger source.

Teacher's Note:
1. Extended sources give umbra and penumbra.
2. A point source gives only umbra.