# How do you define focal length?

## How do you define focal length?

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot.

## What two factors will affect the focal length of a lens?

The principal focal length of a lens is determined by the index of refraction of the glass, the radii of curvature of the surfaces, and the medium in which the lens resides.

Does focal length of lens depends on color of light?

Hint: For different wavelengths of light, the point of convergence i.e. the point of the ray focusing is different, so focal length changes for different colors of light ray and hence the focal length of the lens depends upon the color of the light.

### Does focal length of lens depends on wavelength?

The focal length of the lens varies directly with the wavelength of the light used and as wavelength is inversely related to refractive index, the focal length also varies inversely with it. So, focal length in creases with wavelength but decreases with refractive index and vice versa.

### Which Colour power of lens is more?

For convex lens focal length will be maximum for Red color. Focal length of convex lens is directly proportional to wavelength of light passed. As red light has higher wavelength, it refracts less and has greater focal lens. On the contrary, blue light converges more and has small focal length.

Which Colour has more focal length?

red

#### Which Colour has minimum focal length?

Focal length depend on refractive index. Its different for different colors. Red has highest wavelength and glass has minimum refractive index for red. While violet has lowest wavelength and prism has maximum refractive index in violet.

#### Which Colour will travel faster in air?

Is focal length of lens same for all colors?

The focal length of a lens depends on their refractive index. The refractive index is different for the lights of different colours, and then the focal length of a lens will be different for different lights of different colors. So, the statement is False.

## Does red or blue light have a shorter focal length?

A lens will not focus different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the dispersion of the glass.

## What is red light passing through a convex lens is replaced by light of blue Colour?

When red light is replaced by blue light, wavelength of the light decreases. So, refractive index increases. Finally by the relation :Lens maker’s formula : focal length decreases.

When monochromatic red light is used instead of blue light in a convex lens is focal length will?

Refractive index of red light is less than the refractive index of blue light. So, the focal length of the lens will increase when red light is used. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm.

### How does focal length of a convex lens change when blue light is replaced by red light?

Solution. The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. Thus, when we replace red light with a violet wavelength decrease and hence the focal length of the lens also decrease.

### What will be effect on focal length of convex lens If red colored light is replaced by blue colored light?

The focal length of lens does not change when red light is replaced by blue light.

How does focal length change convex lens?

Solution. The focal length of a convex lens increases as the wavelength increases. This is so because the refractive index decreases with an increase in wavelength of light.

#### How does the focal length of a lens change in water?

Thus, the focal length of the lens will increase. When any lens or glass slab is immersed in a liquid whose refractive index is equal to that of lens, refractive index of lens will be equal to 1 relative to that liquid. The focal length of the lens in this case, will tend to infinity.

#### How does a focal length of a convex lens change it red light is used instead of violet light?

How does focal length of a convex lens change if violet light is used instead of red light. i.e., focal length decreases.

How does wavelength affect focal length?

we know that focal length increases with increase of wavelength of light.

## Under what condition does a Biconvex lens?

Solution. A biconvex lens will act like a plane sheet of glass if it is immersed in a liquid having the same index of refraction as itself. In this case, the focal length 1/f = 0 or f †’ˆž.

## Under what conditions does a convex?

A converging lens produced a virtual image when the object is placed in front of the focal point. For such a position, theimage is magnified and upright, thus allowing for easier viewing.

In which condition the glass lens immersed in a liquid will not be visible?

A lens immersed in transparent liquid is not visible. A lens when immersed in liquid becomes invisible. This happens when the refractive index of the liquid is same as that of the lens material. Under such conditions, no light is reflected by the lens and hence it is not visible.

### Under what circumstances does a combination of two lenses behave like a glass slab?

Convex lens is converging lens while concave lens is diverging lens. If these two types of lens of same power are combined together, the diverging power of one will be cancelled out by the converging power of the other. So the combination will act like a glass block.