Formation of Image by a Concave Mirror and Lens

Formation of Image by a Concave Mirror

When the Object is Placed At Infinity

When an object is placed at infinity, the rays coming from it are parallel to each other. Let us consider two rays, one striking the mirror at its pole and the other passing through the centre of curvature. The ray which is incident at the pole gets reflected according to the law of reflection and the second ray which passes through the centre of curvature of the mirror retraces its path. These rays after reflection form an image at the focus. The image formed is real, inverted and diminished.

The image is

• at F
• Real
• Inverted
• Diminished

When the Object is Placed Beyond C

The two rays which are considered to obtain the image are:

• a ray passing through the centre of curvature
• a ray parallel to the principal axis.

The ray passing through the centre of curvature retraces its path and the ray which is parallel to the principal axis passes through the focus after reflection. These rays after reflection meet at a point between C and F. The image is inverted, real and diminishedThe image is

• Between C and F
• Real
• Inverted
• Diminished

When the Object is Placed at the Center of Curvature

Here we consider the two rays, one parallel to the principal axis and the other passing through the focus. The ray of light which is parallel to the principal axis passes through the focus after reflection. The other ray passing through the focus after reflection emerges parallel to the axis. After reflection these rays meet at the centre of curvature to form an inverted image, which is real and of the same size as the object.

The image is

• At C
• Real
• Inverted
• Same size as object

When the Object is Between C and F

Here we consider a ray of light which is parallel to the principal axis and another ray passing through the focus. The ray which is parallel to the principal axis passes through the principal focus and the ray which passes through the focus after reflection emerges parallel to the principal axis. The reflected rays meet at a point beyond C and the image is real, inverted and magnified.
The image is

• Beyond C
• Real
• Inverted
• Magnified

When the Object is at the Focus

Here, we consider a ray of light which is parallel to the principal axis and another ray passing through the centre of curvature. The ray which is parallel to the principal axis passes through the focus and the ray which passes through the centre of curvature retraces its path. The reflected rays are parallel to each other, and would meet only at infinity i.e., the image is formed at infinity and it is a real, inverted and enlarged.

• The image is at infinity
• Real
• Inverted
• Magnified

When the Object is Between the Pole and the Focus

Here we consider a ray of light which is parallel to the principal axis and another ray which is passing through the centre of curvature. The ray which is passing through the centre of curvature retraces its path and the other ray which is parallel to the principal axis after reflection passes through the focus. These rays appear to meet behind the mirror when the reflected rays are extended backwards. The image is virtual, erect and magnified.

Formation of Image by a Concave Lens

When the Object is at Infinity: 

The image is -

• Formed at F₁

• Erect

• Virtual

• Diminished

When the Object is Placed between O and F:

The image is -

• Formed between O and F₁

• Erect

• Virtual

• Diminished

When the Object is Placed at any Position between O and Infinity:

The image is -

• Formed between O and F₁

• Erect

• Virtual

• Diminished