Light - Formulae

 Sign convention for mirrors and Lens: 

	Mirrors	Lens
Focal Length (f)	Concave mirror – Negative	Concave Lens – Negative
	Convex mirror – Positive	Concave – Positive
Object Distance (u)	Always negative	Always negative
Image Distance (v)	Real image – Negative	Real image – Positive
	Virtual image – Positive	Virtual image – Negative
Object Height (h)	Always negative	Always negative
Image Height (h’)	Real image – Negative	Real image – Negative
	Virtual image – Positive	Virtual image – Positive

To find the nature of the image by using Magnification you need to consider sign of magnification and its number differently:

Magnification	Nature of an image
Positive	Virtual and erect
Negative	Real and inverted
Magnification	Size of an image
Greater than ONE	Enlarged
Equal to One	Same size
Less than ONE	Diminished

  

 Now Moving towards formulae :

For mirror :

Mirrors formula :

 

Where, f = Focal length

            u = Object distance

            v = Image distance

Magnification:

  OR     
Where, h = Object height

            h' = Image height

For Lens :

Lenses formula :

Where, f = Focal length

            u = Object distance

            v = Image distance

Magnification:

   OR  

Where, h = Object height

            h' = Image height

Power of lens :

Refractive Index:

Relative Refractive Index: 

Where, v_{1 = Speed of light in the first medium}

                v_{2 = Speed of light in the second medium}

Absolute Refractive Index : 

where, C = Speed of light in the vacuum ( C = 3 × 10⁸ m/s)

            V = Speed of light in the given medium