Skip to main content

Example questions from refraction through curved surfaces

 

Example questions

1  1) Two converging lenses are to be placed in the path of parallel rays so that the rays remain parallel after passing through both lenses. How should the lenses be arranged? Explain with a neat ray diagram.

ANS:

Two convex lenses of Same focal length


 

i) A parallel beam of light rays will converge on focal point of the lens after refraction.

ii) Light rays passes through the focal point will parallel to principal axis after re-fraction.

iii) So the two lenses are arranged  on a common principal axis such that their focal points coincide with each other, then the rays remain parallel after passing through both lenses.

2    2) The focal length of a converging lens is 20cm. An object is 60cm from the lens. Where will the image be formed and what kind of image is it?

ANS:  Given, f=20 cm

                        U=-60cm

            Lens formula

            1/f=1/v-1/u

            1/20=1/v-(1/-60)

            1/20=1/v+1/60

            1/v=1/20-1/60

            1/v= (3-1)/60  [L.C.M]

            1/v=2/60

            1/v=1/30

Therefore v=30 cm

3       3) A double convex lens has two surfaces of equal radii ‘R’ and refractive index n = 1.5. Find the focal length ‘f’.

ANS:  Given 

Radius of curvature =R

 

focal length =f

 

Refractive index,  μ=1.5

 

From lens maker's formula

 

f1​=(μ−1)[1/R1​-1/R2]

 

f1​=(1.5−1)[1/R+1/R​]

 

=0.5×2/R

 

1/f=1/R

 

f=R

 

4)

Convex lens made with different materials

 

                          

A convex lens is made up of three different materials as shown in the figure. How many of images does it form?

 

ANS: The answer is very simple number of images formed by a lens is equal to the number of different materials used to make the lens.

So, in the above example the number of images formed are three.

 

5)  Find the radii of curvature of a convexo –concave convergent lens made of glass with refractive index n=1.5 having focal length of 24cm. One of the radii of curvature is double the other.

Concavo convex lens problem


thank you


Labels:

Comments

Post a Comment

Please don't post spam links in comments

Popular posts from this blog

Defects of vision and correction

  WHAT HAPPENS WHEN CILIARY MUSCLES CANNOT ACCOMMODATE EYE LENS?   This happens when ciliary muscles become weak and cannot adjust the focal length of the eye lens. This may be due to ageing or some other reasons of lack of vitamins, etc When proper accommodation of eye lens is not done then it leads to defects of eye. We have mainly three defects of vision. They are:   1)     MYOPIA (SHORT SIGHTEDNESS): this defect is due to failure of adjusting maximum focal length of eye lens to 2.5 cm when seeing the far objects. The people with this defect have far point (M). And beyond far point the objects are not seen because f max for these people will be less than 2.5 cm. So image forms in front of retina. So these people can see a blur image of far objects (beyond M). So these people can see the objects only between least distance of distinct vision (L) and far point (M).   Far point: the extreme point from where an object appears clearly t...
Post a Comment
Read more

Working of a battery

  WORKING OF BATTERY: A battery consists of two metal plates (electrodes) and a chemical (electrolyte). The electrolyte (chemical) between the two metal plates consists of positive and negative ions which move in opposite directions The electrolyte exerts a certain force on these ions and makes them move in a specified direction. Let us call this force as a chemical force (Fc). Depending upon the nature of the chemical, positive ions move towards one of the plates and accumulate on that plate.   As a result of this accumulation of charges on this plate it becomes positively charged (called anode). Negative ions move in a direction opposite to the motion of positive ions and accumulate on the other plate. As a result of this the plate becomes negatively charged (called cathode). This accumulation of different charges on respective plates continues till both plates are sufficiently charged.          But the ions in motion experience another force , when ...
Post a Comment
Read more