IEMDaily - Video Lecture Notes

Diagram

Ray diagrams for image formation by lenses

Ray diagram for image formation by lenses is shown in the attached figure.

Diagram

Images formed in different lenses

Definition

Images Formed By Lenses

Image formed by convex lens placed at different distances is shown in the figure

Definition

Images through lenses as real, virtual, erect or magnified

Figure shows the virtual image formed by convex lens and real image formed by concave lens.

Diagram

Different images through lenses

Characteristics of image formed by convex lens is summarized in the table below:

Object position	Image position	Image size	Nature of image	Application
Infinity	$F_{2}$	Point-sized	Real, inverted	Burning glass, To concentrate solar energy on small photovoltaic cells
Beyond $2 F_{1}$	Between $F_{2}$ and $2 F_{2}$	Diminished	Real,, inverted	Camera lens
At $2 F_{1}$	At $2 F_{2}$	Same size as object	Real, inverted	Telescope
Between $F_{1}$ and $2 F_{1}$	Beyond $2 F_{2}$	Magnified	Real, inverted	Slide projector
At $F_{1}$	infinity	Highly magnified	Real, inverted	Spectrometer
Between $F_{1}$ and $O$	On same side as object	Magnified	Virtual, erect	Magnifying glass

Concave lens form virtual and erect image on the same side of the lens between

F_{1}

and

O

. It is always diminished. They are used in Galilean telescope.

Diagram

Characteristics of images by symmetrical lens

Characteristics of image formed by convex lens is summarized in the table below:

Object position	Image position	Image size	Nature of image	Application
Infinity	$F_{2}$	Point-sized	Real, inverted	Burning glass, To concentrate solar energy on small photovoltaic cells
Beyond $2 F_{1}$	Between $F_{2}$ and $2 F_{2}$	Diminished	Real,, inverted	Camera lens
At $2 F_{1}$	At $2 F_{2}$	Same size as object	Real, inverted	Telescope
Between $F_{1}$ and $2 F_{1}$	Beyond $2 F_{2}$	Magnified	Real, inverted	Slide projector
At $F_{1}$	infinity	Highly magnified	Real, inverted	Spectrometer
Between $F_{1}$ and $O$	On same side as object	Magnified	Virtual, erect	Magnifying glass

Concave lens form virtual and erect image on the same side of the lens between

F_{1}

and

O

. It is always diminished. They are used in Galilean telescope.

Result

Image formation by lenses

Characteristics of image formed by convex lens is summarized in the table below:

Object position	Image position	Image size	Nature of image	Application
Infinity	$F_{2}$	Point-sized	Real, inverted	Burning glass, To concentrate solar energy on small photovoltaic cells
Beyond $2 F_{1}$	Between $F_{2}$ and $2 F_{2}$	Diminished	Real,, inverted	Camera lens
At $2 F_{1}$	At $2 F_{2}$	Same size as object	Real, inverted	Telescope
Between $F_{1}$ and $2 F_{1}$	Beyond $2 F_{2}$	Magnified	Real, inverted	Slide projector
At $F_{1}$	infinity	Highly magnified	Real, inverted	Spectrometer
Between $F_{1}$ and $O$	On same side as object	Magnified	Virtual, erect	Magnifying glass

Concave lens form virtual and erect image on the same side of the lens between

F_{1}

and

O

. It is always diminished. They are used in Galilean telescope.

Result

Experimental determination of focal length of lenses

To determine the focal length of convex lens:
Refer the diagram (Figure 1):
The lens is set up in a suitable holder with a plane mirror behind it so that light passing through the lens is reflected back again.
The object used is a hole and cross-wire in a white screen illuminated by a pearl electric lamp.
The position of the lens holder is adjusted until a sharp image of the object is formed on the screen alongside the object itself.
The object will now be situated in the focal plane of the lens, i.e., a plane through the principal focus at right angles to the principal axis
Under these conditions, rays from any point on the object will emerge from the lens as a parallel beam.
They are therefore reflected back through the lens and brought to a focus in the same plane as the object.
The distance between lens and screen now gives the focal length of the convex lens.

To determine the focal length of concave lens (out of contact method):
Arrange the apparatus as shown in the diagram (Figure 2).
The real image

(I_{1})

formed by the convex lens will act as the virtual object for the concave lens. When concave lens is interposed between the convex lens and the real image