Sita, 1·5 m high, stands before a plane mirror fixed on a wall to view her full image. What should be the minimum height of the plane mirror so that Sita can view her image fully?
- A.0·50 m
- B.0·35 m
- C.0·75 m
- D.0·25 m
Correct answer
Explanation
The correct answer is C, 0·75 m. To see one's full image in a plane mirror, the mirror must be at least half the person's height: half of 1·5 m is 0·75 m.
Light from Sita's feet must reflect from the mirror into her eyes, and so must light from the top of her head. By the law of reflection, the ray from the feet meets the mirror at a point halfway between the eye level and the feet, and the ray from the head meets it halfway between the head and the eyes. The mirror therefore needs to span half the distance from eyes to feet plus half the distance from eyes to head, which is half her height. The result does not depend on how far she stands.
A is wrong: 0·50 m is one-third of her height and would cut off part of the image.
B is wrong: 0·35 m is far less than half her height.
D is wrong: 0·25 m would show only a small part of her body.
Exam tip: minimum plane-mirror height for a full image = half the person's height, whatever the distance.
Practice Questions
View allConsider the following statements :
Statement-I : Giant stars live much longer than dwarf stars.
Statement-II : Compared to dwarf stars, giant stars have a greater rate of nuclear reactions.
Which one of the following is correct in respect of the above statements?
- A.Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- B.Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
- C.Statement-I is correct, but Statement-II is incorrect
- D.Statement-I is incorrect, but Statement-II is correct
Show answer
Massive giant stars burn their fuel so fast that they live much shorter lives than dwarf stars, so Statement I is false and Statement II is true.
Read the full explanation →Which one of the following is a filter feeder?
- A.Catfish
- B.Octopus
- C.Oyster
- D.Pelican
Show answer
The oyster feeds by drawing water over its gills and straining out plankton and organic particles, which is filter feeding.
Read the full explanation →With reference to radioisotope thermoelectric generators (RTGs), consider the following statements :
1. RTGs are miniature fission reactors.
2. RTGs are used for powering the onboard systems of spacecrafts.
3. RTGs can use Plutonium-238, which is a by-product of weapons development.
Which of the statements given above are correct ?
- A.1 and 2 only
- B.2 and 3 only
- C.1 and 3 only
- D.1, 2 and 3
Show answer
RTGs power spacecraft using heat from the natural decay of plutonium-238; they are not reactors and involve no fission. In an RTG, a radioactive isotope gives off heat as it decays on its own.
Read the full explanation →Consider the following :
1. Cashew
2. Papaya
3. Red sanders
How many of the above trees are actually native to India ?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
Of the three, only red sanders is native to India; cashew and papaya were brought from the Americas. Red sanders (Pterocarpus santalinus) is endemic to the southern Eastern Ghats, mainly the Seshachalam and nearby hills of Andhra Pradesh.
Read the full explanation →Consider the following :
1. Butterflies
2. Fish
3. Frogs
How many of the above have poisonous species among them ?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
Each of the three groups includes species that carry toxins in their bodies to deter predators. Butterflies: the Monarch butterfly feeds as a caterpillar on milkweed and stores its bitter cardiac glycosides, so birds that eat it fall sick; its bright orange colour is a warning.
Read the full explanation →