Q. Why does we get dew on the surface of cold soft drink bottle kept in open air?
A: It is because of condensation of water molecules in the air on the surface of cold drink bottle.
1. When the water molecules in air, during their motion strike the surface of cold soft drink bottle, they loose their kinetic energy.
2. When the water molecules loose their energy their temperature falls down and so they condense as water droplets on the surface of the
bottle.
Q. Why do stars appear twinkling?
A: We view the stars from earth through thick layers of turbulent air of the earth’s atmosphere.
1. The light from the stars reach the viewer after passing through these layers of air. It is refracted many times in random directions.
2. These random refractions make the star to appear twinkling to the viewer.
3. A viewer cannot experience this type of vision of twinkling if he could see the stars beyond the atmosphere of the earth.
Q. How do you appreciate the role of Fermat principle in drawing ray diagrams?
A: According to Fermat’s principle that light selects the shortest path to travel.
1. This is the reason for the propagation of light in straightlines.
2. We use this principle, when we draw ray diagrams to find the image of an object formed by mirrors to illustrate the laws of reflection, refraction etc.
Q: What are optically denser and rarer media?
A : Let V1 and V2 be the speed of light in the two given media respectively.
1. If V1 > V2 then the first medium is denser medium and the second medium is rarer medium.
2. If V2 > V1 then the second medium is denser medium and the first medium is rarer medium.
Q: Suppose you are inside the water in a swimming pool near an edge. A friend is standing on the edge. Do you find your friend taller or shorter than his usual height?
A : The friend seems to be taller than his actual height.
1. This is because the light ray travels from a rarer medium (air) to denser medium gets refraction and bends towards the normal.
2. This seems to be coming from a distant point for the swimmer. So the person appears taller than his usual height.
Practice Questions – 10th Physical Science.
1 . The focal length of a converging lens is 20 cm. An object is 60 cm. from the lens. Where will the image be formed and what kind of image is it?
2. If a white sheet of paper is stained with oil, the paper turns transparent. Why?
3. What do you mean by power of a lens?
4. Why is the sky blue?
5. Why does the picture appear distorted when a bar magnet is brought close to the screen of a television.
6. Give few applications of Faraday’s law of induction in our daily life.
7. What is the magnetic force on the charge moving parallel to a magnetic field?
8. List three metals that are found in nature in uncombined form?
9. What is the difference between roasting and Calcination. Give one example for each.
10. How many water molecules are present in the following ores:
a) Bauxite b) Epsom salt c) Gypsum d) Carnallite.
11. Give an example for esterification reaction?
12. Suggest a chemical to distinguish between ethanol and ethanoic acid and explain the procedure.
13. What is soap?
14: State and explain Hund’s rule taking the configuration of carbon as an example.
15: What is an electromagnetic spectrum? Give a familiar example for it.