Question

how will you find focus of convex lens

Answer 1

Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is aconverging lens. When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.
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Answer 2

DEFINITION 1:

The focal point of a convex lens is the point where light rays parallel to the axis are brought to a point. The distance from the lens to this point is called the focal length of the lens.



Because it seems rather odd to represent light as a dark line on a white page, the diagram above has been inverted below to show white light on a black background. The principle is the same.



Now the question is where one would find parallel light rays in nature? How common or uncommon are parallel light rays if most of the light we seen on a daily basis is diverging to one degree or another?

If an object is very far away, the angle formed between adjacent light rays is very small. Depending on the focal length of the specific lens, this distance might be anywhere from a few meters to a kilometer. If the object is very far, say 93,000,000 miles (1.5 x 1011 m) like the Sun, the distance is sufficiently far that light rays are essentially parallel. So sunlight is a convenient source of parallel light rays. Objects that are a great distance away like hills or trees may also furnish almost parallel rays. Finally, lasers are a relatively inexpensive source of parallel light due to their inherent nature.

NOTE: The light rays do not stop when they get to the focal point. They just happen to pass through this point and continue their journey on into the universe.

NOTE 2: In the diagrams above, light rays are shown bending at the center of the lens. This is a construction technique and is used only for convenience. In fact the rays would bend once upon entering the lens and a second time upon exiting.

BOTTOM LINE: If we see a light ray that's parallel to the axis of a convex lens we know where it is going to go on the other side -- through the focal point.

 

DEFINITION 2:

Diverging light rays striking a convex lens can be bent until they emerge parallel to the axis. The point where this happens is called the focal point.



Or as before, white light on a black background:



NOTE: Because we have defined "focal point" so precisely, we can understand that a light ray that is not parallel to the axis will not pass through the focal point on the other side of the lens. Also we know that a light ray that does not pass through the focal point will not emerge parallel to the axis on the other side.

BOTTOM LINE: If we have a light ray that either starts at the focal point, passes through the focal point or looks to the lens like it starts at the focal point, that light ray will be bent until it is parallel to the axis.

 

BIG NOTE:

A convex lens has two focal points - one on each side. They are equal distances from the lens. The lens does not have to have the same curvature on both sides for this to be true, and it doesn't depend on the direction the light takes entering the lens. It is the combined curvature that determines the focal point.