CBSE Class 7 Science Advanced Physics Light Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. Glow-worm is a ________ source of light.
Answer:
Glow-worm is a cold (natural) source of light.
In simple words: It makes light without being hot.

Teacher's Note:
1. A cold source gives light even at normal temperature.
2. A glow-worm's light is also called bioluminescence.

Question 2. When the Moon comes into the umbra and penumbra of the Earth, ________ is formed.
Answer:
Lunar eclipse is formed.
In simple words: The Earth's shadow falls on the Moon.

Teacher's Note:
1. Lunar eclipse needs a full-moon night.
2. The Earth lies between the Sun and the Moon.

Question 3. The phenomenon of light involved in the formation of shadow is ________.
Answer:
Rectilinear propagation of light.
In simple words: Light going in straight lines makes shadows.

Teacher's Note:
1. Because light cannot bend around an opaque object, a dark region forms behind it.
2. Eclipses depend on the same property.

Question 4. Periscope works on the principle of ________ of light.
Answer:
A periscope works on the principle of reflection of light.
In simple words: Two mirrors turn the light twice.

Teacher's Note:
1. Each plane mirror reflects the light through 90°.
2. Some periscopes use prisms and total internal reflection.

Question 5. The image formed by a plane mirror is ________ in nature.
Answer:
The image is virtual in nature.
In simple words: You cannot catch it on a screen.

Teacher's Note:
1. The reflected rays only appear to come from behind the mirror.
2. It is also erect and laterally inverted.

Question 6. Real image is formed by a ________ mirror.
Answer:
A real image is formed by a concave mirror.
In simple words: Only a concave mirror can bring light together.

Teacher's Note:
1. A real image needs reflected rays to actually meet.
2. Convex and plane mirrors give virtual images only.

Question 7. The image of an object formed by the inner surface of a spoon, when it is placed very close to it, is ________ and ________.
Answer:
The image is virtual and erect.
In simple words: A close object in a concave mirror gives a big, upright, virtual image.

Teacher's Note:
1. The inner surface of a spoon is a concave mirror.
2. Object between pole and focus gives a magnified virtual image.

Question 8. 1 watt = ________ lumen.
Answer:
1 watt = 683 lumen.
In simple words: Just a number from the chapter.

Teacher's Note:
1. Also remember 1 lumen = 12.56 candle power.
2. Do not mix it up with 10.76 for foot candle.

Question 9. Two plane mirrors are inclined at an angle of 30°, the number of images formed are ________.
Answer:
Number of images = 360°/30° = 12, which is even.
So n = 12 − 1 = 11.
In simple words: Twelve minus one gives eleven.

Teacher's Note:
1. If 360/θ is even, subtract 1.
2. Check: for 90° we get 3 images.

Question 10. Moon is a luminous object. (True/False)
Answer:
False. The Moon does not produce light; it reflects sunlight.
In simple words: The Moon shines only because of the Sun.

Teacher's Note:
1. Luminous bodies make their own light.
2. The Moon is non-luminous.

Question 11. Eclipses are formed due to rectilinear propagation of light. (True/False)
Answer:
True. Light travels in straight lines, so the Moon or the Earth can cast a shadow on the other.
In simple words: Straight-line light gives eclipses.

Teacher's Note:
1. Lunar and solar eclipses are both shadow effects.
2. Think of the Sun, Earth and Moon in a straight line.

Question 12. The path of light is a straight line. (True/False)
Answer:
True. In a homogeneous medium light travels in a straight line.
In simple words: Light goes straight in one uniform medium.

Teacher's Note:
1. This is rectilinear propagation.
2. It bends only at the boundary of two media.

Question 13. Sun light is not necessary for the growth of plants. (True/False)
Answer:
False. Plants need light for photosynthesis.
In simple words: No light, no food for the plant.

Teacher's Note:
1. Chlorophyll converts light energy into chemical energy.
2. Plants kept in the dark become weak and pale.

Question 14. A virtual image can be obtained on a screen. (True/False)
Answer:
False. A virtual image cannot be caught on a screen; only a real image can.
In simple words: Only real images show up on a screen.

Teacher's Note:
1. Virtual images are formed by diverging rays.
2. Plane mirror images are virtual.

Question 15. The rear-view mirror used in vehicles is a convex mirror. (True/False)
Answer:
True. A convex mirror gives a wide field of view.
In simple words: Convex mirrors let drivers see more of the road behind.

Teacher's Note:
1. It forms a small, erect, virtual image.
2. The wide view is the main reason.

Question 16. If the Sun is in the west, a rainbow is formed in the east. (True/False)
Answer:
True. A rainbow is seen in the direction opposite to the Sun, because sunlight is dispersed by raindrops in front of the observer who has the Sun behind.
In simple words: Rainbows appear opposite the Sun.

Teacher's Note:
1. The observer has the Sun at the back.
2. Rain should be present in front.

Question 17. The sky appears blue due to scattering of light. (True/False)
Answer:
True. Air molecules scatter blue light most, so we see a blue sky.
In simple words: Blue light is scattered the most by the air.

Teacher's Note:
1. Scattering means light is sent in all directions.
2. This is explained more in higher classes.

Question 18. Oil paper is an example of ________ medium of light.
Answer:
Oil paper is a translucent medium.
In simple words: It lets only some light pass.

Teacher's Note:
1. Translucent media pass part of the light.
2. Fog is another example.