Question
Write a note on twinkling of stars.
✓
Answer
- Twinkling of stars is due to atmospheric refraction of starlight.
- Starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.
- As the optical density of air increases towards surface of the earth, light from the star travels from rarer to denser layers, bending every time towards the normal.
- On producing the final refracted ray backwards as shown in the following figure, we find that the apparent position $B$. of a star is higher than the actual position A. of the star as shown in figure.
[Apparent star position due to atmospheric refraction]
- The star appears somewhat higher (above) than its actual position when viewed near the horizon.
- Further, this apparent position of the star is not stationary, but keeps on changing slightly, since physical conditions of the earth’s atmosphere are not stationary.
- Since the stars are very distant, they appear as point-sized sources of light.
- Due to a continuous change in the direction of propagation of light, the apparent position of the star fluctuates all the time and the amount of starlight entering, the eye flickers, i.e., the brightness of the star changes continuously (the star sometimes appears brighter and at some other time fainter).
- This is called the twinkling of a star.
Need a full question paper?
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.
Explore more
Similar questions
Answer the following questions regarding blood pressure :
(1) What is called blood pressure?
(2) What is called systolic and diastolic pressure?
(3) What is the normal value of systolic and diastolic pressure?
(4) Which instrument is use to measure blood pressure?
(5) What is hypertension? How it is caused?
(1) What is called blood pressure?
(2) What is called systolic and diastolic pressure?
(3) What is the normal value of systolic and diastolic pressure?
(4) Which instrument is use to measure blood pressure?
(5) What is hypertension? How it is caused?
What is a spherical mirror? State its types.
Give reasons.
$(a)$ Platinum, gold and silver are used to make jewellery.
$(b)$ Sodium, potassium and lithium are stored under oil.
$(c)$ Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
→$(a)$ Platinum, gold and silver are used to make jewellery.
$(b)$ Sodium, potassium and lithium are stored under oil.
$(c)$ Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
Explain in details about small intestine.
An object is placed at a distance of $10 \ cm$ from a convex mirror of focal length $15 \ cm$. Find the position and nature of the image.
→What is meant by amphoteric oxide? Give two examples of it and write the equation of its reaction with acid and base.
Name and state the rule which determine the direction of magnetic field around a straight current-carrying conductor. Also draw diagram
What is a free electron? Explain conducting and nonconducting materials in terms of it.
Write short notes: Alveoli of lungs.
Your neighbour is a chain-smoker. He often suffers from cough and lung infection. His relatives often tell him to leave smoking.
(1) How inhaled air is filtered in the upper part of respiratory tract ?
(2) Which is the effect of smoking on the upper part of respiratory tract?
(3) Why do you call smoking as injurious to health?
(1) How inhaled air is filtered in the upper part of respiratory tract ?
(2) Which is the effect of smoking on the upper part of respiratory tract?
(3) Why do you call smoking as injurious to health?