Micro-organisms friend and foe — Science STD 8 — Question

State one way in which the planets can be distinguished from the stars in the night sky.

What is a vibration (or an oscillation)? Define 'amplitude' of vibration of an object.

Name an eye ailment (or eye-disease) caused by the deficiency of vitamin A in the diet.

What is the angle of incidence when a ray of light is incident normally on a plane mirror?

The farmers use large amounts of a substance P in the fields to increase the crop yield. The excess of P dissolves in water and runs into a lake. The substance P causes rapid growth of tiny green water plants Q in the lake which cover the whole lake like a green sheet. When the plants Q die, the organisms called R decompose them by utilizing S dissolved in lake water. The amount of dissolved Sin water decreases too much due to which the fish living in lake suffocate and die. What are P, Q, R and S?

Name the point inside the human eye where the image is not visible.

What are rods and cones in the retina of an eye?

A positively charged object repels another charged object kept close to it. What is the nature of charge on the other object?

Which effect of electric current is utilized for detecting the flow of current through a solution:

1. When a torch bulb is used?
2. When a compass is used?

Why do mountaineers usually suffer from nose-bleeding at high altitudes?