# Why the focal length of convex lens is positive and concave lens is negative?

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## Why is the focal length of a concave lens negative?

The power of the lens is the reciprocal of the focal length. A concave lens is a diverging lens, so it will always have a negative focal length. Therefore, the power of a concave lens is also negative.

## Why is the focal length of concave mirror is taken as negative and that of convex mirror is taken as positive?

Since, object is always placed in front of the mirror hence the sign of object is taken as negative. Since, the centre of curvature and focus lies behind the convex mirror, so sign of radius of curvature and focal length are taken as + (positive) in the case of convex mirror.

## Why focal length of convex lens is positive?

Convex lens converges incident rays. Hence the rays to converge at a point in front of the lens. So image distance is positive here.

## Is the focal length of a concave lens negative or positive?

The focal length of a concave surface is negative, whereas the focal length of a convex surface is positive. Therefore, for a lens, the power of a concave and convex lens is negative and positive respectively. Whereas, for a mirror, the power of concave and convex lens is positive and negative respectively.

## Is V positive in convex lens?

v’ is always positive for convex mirror & concave lens. Distances measured from the optic center to the right of the lens are considered +ve and t from optic center to the left are -ve.

## Can focal length of concave mirror be positive?

Focal length formula for mirror is 1f=1v+1u, where vis the distance from pole to image and u is the distance from pole to object. Here we use sign conventions. Focal length is positive for a concave mirror and negative for convex mirror.

## What is the focal length of concave mirror?

Focal Length – The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a ‘Real image’ of a distant object at its focus, as shown in the figure.

## Which lens has positive magnification?

Concave lenses always form virtual images, so magnification produced by a concave lens is always positive. A concave lens always forms the image smaller than the object, so |m|<1.

## What is the formula of focal length of convex lens?

According to the convex lens equation, 1/f = 1/v + 1/u. It relates the focal length of a lens with the distance of an object placed in front of it and the image formed of that object. 2.

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## How do you know if an image is positive or negative?

When the image distance is positive, the image is on the same side of the mirror as the object, and it is real and inverted. When the image distance is negative, the image is behind the mirror, so the image is virtual and upright. A negative m means that the image is inverted. Positive means an upright image.

## What is the difference between positive and negative lenses?

A positive lens is one that causes incident parallel rays to converge at a focal point on the opposite side of the lens. A negative lens is one that causes incident parallel rays to emerge from the lens as though they emanated from a focal point on the incident side of the lens.

## How do you know if an object is positive or negative?

Sign convention

In other words, if the image is on the far side of the lens as the object, the image distance is positive and the image is real. If the image and object are on the same side of the lens, the image distance is negative and the image is virtual. For converging mirrors, the focal length is positive.