Focal Length Of Concave Mirror And Convex Lens Class 10th Science Lab Manual CBSE Solution

Class 10th Science Lab Manual CBSE Solution

Lab Experiment 10a
Question 1.

AIM

To Determine the focal length of the concave mirror by obtaining the image of a distant object.


Answer:

MATERIALS REQUIRED


A concave mirror, a metre scale, a mirror holder.


THEORY


1. A spherical mirror whose reflecting surface is curved inwards, i.e., faces towards the centre of the sphere is called a concave mirror.


2. It obeys the law of the reflection of light like a plane mirror.


3. Thus, the parallel beam of a ray of light coming from a distant object, such as the sun or a building can be considered as parallel.


4. When the parallel ray is incident on the reflecting surface of a mirror, then after reflection, the rays converge at a point, and this point is called principal focus of the concave mirror as shown in the figure.


5. since the image formed by the mirror is real so that it can be obtained on the screen.


6. The distance between the pole O and principal focus F of a concave mirror is called the focal length of the mirror. It is equal to half the radius of curvature of the mirror.



PROCEDURE


1. Choose a distant object like a tree or the sun to at as an object for our experiment.


2. Mount the concave mirror in a mirror holder.


3. Adjust the concave mirror in such a way that the rays of light coming from the tree fall directly on its reflecting surface.


4. Measure the horizontal distance between the wall and the centre of a concave mirror with the help of a meter scale.



5. Repeat the experiment by selecting the different distant objects at different distances to measure the focal length of the concave mirror.


OBSERVATION AND CALCULATION



Mean Focal length = 18 cm


RESULT


The approximate focal length of the given concave mirror is 18 cm as determined by the above method.

According to the sign conventions, the focal length of a concave mirror is negative. Therefore,
f = - 18 cm


PRECAUTIONS


1. The mirror should be placed vertically in the lens holder.


2. There should not be any obstacle in the path of rays of light incident on the concave mirror


3. The meter scale must be correctly positioned between the wall and centre of the concave mirror.



Lab Experiment 10b
Question 1.

AIM

To determine the focal length of a convex lens by obtaining the image of a distant object.


Answer:

MATERIALS REQUIRED


A convex lens, a lens holder, a white screen such as a wall or white painted board, meter scale, distant object.


THEORY

1. A ray of light passing through the first principal focus or appearing to meet at it emerges parallel to the principal axis after refraction.
2. Convex lens converges the incident parallel beam of light on the other side of the lens.
3. T image formed by the convex lens may be real or virtual, inverted or erect, diminished, the same size or magnified depending on the various positions of the object.
4. Focal length for the convex lens is positive while it is negative for the concave lens.
5. The lens behaves like a plane glass plate when it is kept in a medium whose refractive index is equal to that of the lens.


PROCEDURE


1. Choose any distant object like a tree or the sun as an object for the experiment.


2. Held the lens vertically inside a lens holder and lens must be kept in vertical position during the experiment.


3. Place a screen on the other side of the lens.


4. Obtain the image of tree/building on a white screen or wall. Move the lens to get a sharp image.


5. Using a meter scale, measure the distance from the lens to the screen.


6. Repeat this procedure by changing the object.


OBSERVATION AND CALCULATION



Mean value of the focal length = 13.25 cm


RESULT



The approximate focal length of a convex lens = 13.25 cm.


PRECAUTIONS


1. The lens should be kept vertical inside the lens holder.

2. While measuring the distance, the distance between the sharp image and the centre of the lens is to be measured.

3. The lens the screen must be at the same level.



Viva Questions
Question 1.

What is a spherical mirror, give its type ?


Answer:

The spherical mirror is a part of a hollow sphere with one side having silver/mercury coating, further coated with paint to protect it from damage.

It is of two types :


1. Concave mirror: Silvered at the outer surface so that reflection takes place from the inner surface(concave).


2. Convex mirror: Silver at inner surface so that reflection takes place from the outer surface(convex).



Question 2.

What is an aperture of the spherical mirror?


Answer:

It is the width of the reflecting surface from which reflection takes place.



Question 3.

what is centre and radius of curvature?


Answer:

, The centre of curvature, is the geometrical centre of the hollow sphere “c” from which the mirror is formed.

The radius of curvature is the radius “R” of the hollow sphere whose part is a spherical mirror.



Question 4.

Define principal focus and axis?


Answer:

The Principal axis is the straight line joining the pole and centre of curvature.

The principal focus is the point “f” on the parallel axis where a parallel beam of light actually meets after reflection.



Question 5.

Where is the brightest and sharpest image is formed by a concave mirror ?


Answer:

The brightest and the sharpest image for the concave mirror is formed at the focus.



Question 6.

Comment on the image formed by a concave mirror?


Answer:

1. Virtual, erect and enlarged image of an object is formed behind the concave mirror.


2. The real and inverted image is formed in front of the mirror.



Question 7.

Why concave mirror is called a converging mirror?


Answer:

A concave mirror is called a converging mirror because it converges the parallel beam of light ray after reflection at a point.



Question 8.

Give two uses of the convex mirror?


Answer:

The uses of concave mirror are:

1. They are reflectors in the torch to reflect the light rays.


2. It is used as a shaving mirror to get an erect and enlarged image of the face.



Question 9.

What is the difference between a real and virtual image?


Answer:

A real image is obtained on the screen, they are formed by the interaction of a ray of light after reflection.

A virtual image is formed when the ray of light appear to meet after the reflection; Image cannot be taken on the screen.



Question 10.

State law of reflection of light ?


Answer:

1. The angle incidence is equal to the angle of reflection.


2. The incident ray, the reflected ray and the normal at the point of incidence,all lie on the same plane.



Question 11.

What is a lens?


Answer:

A homogenous transparent material or medium bounded by two surfaces at different or same radii of curvature is called a lens.



Question 12.

What are different types of lens?


Answer:

1. Double convex lens: if both the refracting surface is convex, it is thicker at the middle and thinner at the edges.


2. Double concave lens: If both the refracting surface of the lens is concave, it is thinner at middle and thicker at the edges.



Question 13.

Write the lens formula?


Answer:

Lens formula:


Where f is the focal length of the lens


u is the distance between the object, and the lens


v is the distance between the image and the lens.



Question 14.

Write the mirror formula?


Answer:

The mirror formula:


Where f is the focal length of the mirror


v is the distance between the image and lens


u is the distance between the object and the mirror.



Question 15.

When does a lens behave like a plane glass plate?


Answer:

The lens will behave like a plane glass only when it is kept in a medium whose refractive index is equal to that of the lens .



Question 16.

What is the way to increase the magnification power of a lens ?


Answer:

The magnification power of the lens can be increased by using a number of lenses.



Question 17.

Comment on the magnification sign for the concave and convex lens?


Answer:

1. Convex lens, magnification is positive for virtual image and negative for real image.


2. Concave lens, magnification is always positive because virtual image is always formed.



Question 18.

What is the power of the lens?


Answer:

The power lens is the degree of convergence or divergence of the light ray incident on any refracting surface of the lens.



Question 19.

What is a factor the power of lens?


Answer:

the magnification power of the lens is inversely proportional to the focal length of the lens.



Question 20.

If we cover one half of the lens while focusing on a distant object, in what ways will it affect the image formed?


Answer:

A full-size image is formed, but only the intensity or brightness of the image will reduce.


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