Light: Mirrors and Lenses — Science STD 8 — Question

Why are microwave cookwares made of plastic?

Two plane mirrors are set at right angles to each other. A coin is placed in-between these two plane mirrors. How many images of the coin will be seen?

Discuss the effects of deforestation on the following. Cities (Urban areas).

A group of students is experimenting. They have two beakers, A and B, each containing 100 mL of water. In beaker A, they add 5 grams of salt and stir. In beaker B, they add 20 grams of salt and stir. They observe that all the salt in beaker A dissolves, but in beaker B, some salt remains at the bottom, even after stirring vigorously.
Q.1. Why did all the salt dissolve in beaker A, but not in beaker B?
Q.2. What kind of solution is formed in beaker A?
Q.3. What about beaker B (considering the undissolved salt)?

State one advantage of maintaining Red Data Book.

Given below is a conversation between two farmers Heera and Beera.
Heera: Brother Beera, your maize crops look beautiful. They have grown pretty fast.
Beera: Yes, I have sprayed urea this time. What about you ?
Heera: Well, I am still relying on good old cow dung. I am saving money for buying a tractor.
Beera: That's good. Tractor saves a lot of time and labour.
Heera: Yes, it's been very labour intensive for me and now these weeds have come up.
Beera: Try weedicides, they are very effective.
Now, answer the following questions.
(i) List the practices which are not environment friendly and why ?
(ii) What is the advantage of modern agricultural Implements over traditional ones ?
(iii) Name one weedicide and the precaution, farmers must take during its application.

What is acid rain? How is acid rain caused? What are the harmful effects of acid rain?

What is the purpose of establishing several Biosphere Reserve, Wildlife Sanctuaries and National Parks in India?

Explain why, an owl can see well in the night (but not during the day) whereas an eagle can see well during day (but not at night).