Determination of Focal Length of Concave Mirror and Convex Lens

What is Concave Mirror and Convex Lens?

Concave Mirror

A concave mirror, which is also known as a converging mirror, consists of a reflecting surface that has been recessed inward (away from the incident light). The concave mirror can reflect the light inward to one focal point. They are used for focusing light. In a concave mirror, the image type depends on the distance between the object and the mirror.


Convex Lens

A convex lens is a type of lens that is thicker at the center and thinner at the edges. An optical lens is built using two spherical surfaces which if bent outwards will be considered as a convex lens. These types of lenses are used for converging a beam of light and focusing it to a point at the other side.


How to Find the Focal Length of a Concave Mirror - Class 12?

The following are the ways by which students can find the focal length of a concave mirror using a convex lens.

  • A concave mirror can be termed as a spherical mirror consisting of a reflecting surface that is curved inwards, and it also follows the laws of reflection of light.

  • The light rays that are coming from a distant object are considered to be parallel to each other.

  • If the image formed is real, inverted, and very small in size, then it is believed that the rays of light which are parallel to each other meet the point in the front of the mirror.

  • The image that is formed by the concave mirror is considered to be real and can be obtained on the screen.

  • ‘f’ is used to express the difference between the principal axis P and the focus F of the concave mirror.


Materials Required to Find the Focal Length of Concave Mirror Using Convex Lens

The following are the list of materials that are required to find the focal length of a concave mirror using a convex lens.

  • A concave mirror

  • A measuring scale

  • A screen holder

  • A mirror holder

  • A mirror stand  


Procedure to Find the Focal Length of Concave Mirror Using Convex Lens

  • The distance between the distinct objects selected should be more than 50ft.

  • The object and the concave mirror that is placed on the mirror stand should be facing each other.

  • The screen must be fixed in front of the reflecting surface of the mirror in order to obtain a clean and sharp image.

  • The distance between the screen and the concave mirror can be determined by using a meter scale. This distance is the same as the focal length of the given concave mirror.

  • This above process should be repeated three times to obtain the average of the focal length.   


How to Find the Focal Length of a Convex Mirror Using a Convex Lens?

The following are the ways that should be followed to find the focal length of a convex mirror using a convex lens.

  • The convex mirror is thicker in the middle and is thinner at the edges. It is also known as a converging mirror.

  • The refracted rays that are coming from the parallel beam of light converge on the other side of the convex mirror.

  • The image that is obtained by the focus of the lens will be real, inverted, and very small in size.

  • ‘F’ is known as the focal length, which is considered as the distance between the optical center of the lens and the principal focus.

  • There is a possibility of the image being formed on the screen if the image formed by the mirror is real.


Materials Required to Find the Focal Length of Convex Mirror Using a Convex Lens

The following are the list of materials that are used to find the focal length of a convex mirror using a convex lens.

  • A wooden bench

  • A convex lens

  • A lens holder

  • A screen fixed to a stand

  • A measuring scale


Procedure to Find the Focal Length of a Convex Mirror Using a Convex Lens

  • Try to arrange the lens and screen on the wooden bench without disturbing them.

  • The lens should be placed in a holder facing the distant object.

  • The holder should be placed with the screen on the bench.

  • The position of the screen should be such that the sharp image of the distant object is obtained easily without any difficulties.

  • The difference between the screen and the position of the lens is considered to be the focal length of the convex mirror.

  • After completing the above steps, shift the focus towards other distant objects in order to calculate the focal length of the convex mirror.


How is the Concave Mirror Different from a Convex Mirror?

The concave and convex mirrors are different from each other and therefore, give different results. If the outer surface of a spherical mirror is painted then it is known as a concave mirror whereas if the inner surface of a spherical mirror is painted, then it is known as a convex mirror. The concave mirror and convex mirror are also known as the converging mirrors and the diverging mirrors respectively. In the case of the concave mirror, only the inner surface is reflexive whereas, in the case of a convex mirror, only the outer surface is reflexive.


In the case of the concave mirror, the focal length is positive whereas the focal length is negative in the case of a convex mirror. Concave mirrors can form both real and virtual images and thus can be obtained on the screen whereas convex mirrors can form only virtual images which cannot be obtained on the screen. The center of curvature and the reflecting surface fall on the same side in a concave mirror whereas in the case of a convex mirror the center of curvature and the reflecting surface fall on the opposite sides. Vehicle headlights, shaving mirrors, and torches use concave mirrors whereas convex mirrors are used as rear view mirrors in bikes and cars. Magnified images of the object are obtained from a concave mirror whereas a convex mirror provides a wider view of the object.


Conclusion

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FAQs on Determination of Focal Length of Concave Mirror and Convex Lens

1. Differentiate between the convex lens and the concave lens.

Convex Lens

Concave Lens

The focal length of a convex lens is always calculated as positive.

The focal length of a concave lens is always calculated as negative.

Images obtained by the convex lens can be real or virtual.

The image obtained by the concave lens is always virtual.

In a convex lens, the light rays are seen to be converging towards the principal axis.

In a concave lens, the light rays are seen to be diverging from the principal axis.

This lens is used for correcting Myopia.

This lens is used for correcting hypermetropia.

2. Discuss the types of reflection.

There are two types of reflection: regular reflection and irregular reflection.


Regular Reflection

This type of reflection is formed by a plane mirror having a smooth surface. In the case of a regular reflection, the image formed is clear and very much visible. The image formed here will be virtual as it is formed by a plane mirror.


Irregular Reflection

As we know that most of the natural surfaces are rough on the scale of the wavelength of light due to which the incident lights which are parallel to each other are reflected in several directions irregularly.

3. What are the precautions that I should take while determining the focal length of a given concave mirror?

Determination of the focal length of a given concave mirror is a simple laboratory experiment that you can perform provided you keep these following precautions in mind.

  • The distant object should be well illuminated in order to get the image of a distinct object.

  • The placement of the concave mirror should always be done near an open window.

  • The distant object and the polished surface of the concave mirror should always face each other.

  • The rays of light from the object to the concave mirror should pass smoothly without any hurdle or hindrances.

  • The stand of the concave mirror, as well as the screen, should be placed parallel to the measuring scale.

  • Also, ensure that the mirror along with the mirror holder should be placed perpendicular to the measuring scale.

4. What are the precautions that I should take while determining the focal length of a given convex lens?

Determination of the focal length of a given convex lens is a simple laboratory experiment that you can perform provided you keep these following precautions in mind.

  • You should ensure that you have placed the convex lens vertically.

  • The rays of light passing from the object and the convex lens must pass smoothly without any hindrance or hurdle.

  • The object that is distant from the lens must be placed in a well-illuminated area to get a distinct image of the distant object.

  • The screen and the stand on which the convex lens is placed should be placed parallel to the measuring scale.

5. What is the theoretical aspect behind finding the focal length of a concave mirror and a convex lens?

In order to find the focal length of a concave mirror, you must know the following theoretical aspects of the mirror:

  • A mirror that has a reflecting surface inwards is known as a concave mirror and such mirrors follow the laws of reflection.

  • The light rays that come from a distant object are always parallel to each other.

  • If the parallel rays of light meet at a point in the front of the mirror, then the image formed will be very small in size, real and inverted, that is, these images can be obtained on the screen.

  • The difference between the focus F and the principal axis ‘P’ of the concave mirror denotes the focal length, that is, ‘f’.

In order to find the focal length of a convex lens, you must know the following theoretical aspects about the lens: 

  • The converging lens also commonly known as the convex lens has thinner edges while the middle part of the lens is thicker.

  • The parallel beam of light which is reflected by the lens converges on the other side of the convex lens.

  • If the converged rays of light meet at the focus of the lens, then it will form inverted, real, and very small images, that is, such images can also be obtained on the screen.

  • The difference between the principal focus and the optical center of the lens determines the focal length of the lens which is determined by ‘f’.

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