Human Eye and Colourful World — Science STD 10 — Question

What is placenta? State its function in human female.

Why we have two eyes to view the objects?

What is meant by positive phototropism and negative phototropism? Give one example of each type.

Describe how, the water of river Ganga has been highly polluted.

Describe the working of the human eye with the help of the above diagram.

Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an observer on the moon? Justify your answer with a reason.

An ore on treatment with dilute hydrochloric acid gives a smell like that of rotten eggs. What type of ore is this? How can it be concentrated? How can the metal be obtained from the concentrated ore?

A dentist’s mirror has a radius of curvature of 3cm. How far must it be placed from a small dental cavity to give a virtual image of the cavity that is magnified five times?

An object is placed at a distance of 100cm from a converging lens of focal length 40cm.

1. What is the nature of image?
2. What is the position of image?

A convex lens produces an inverted image magnified three times of an object placed at a distance of 15cm from it. Calculate focal length of the lens.

If one hold his breath after expiration for about 30sec., would there still be occurring any exchange of respiratory gases in the lung during this period? Explain.