# Intensity of light

1. Nov 9, 2007

### pardesi

What do we mean in mathematical terms intensity of light at this point is $$I$$ and when we say Power of light is $$P$$

2. Nov 9, 2007

### pixel01

the power of light P means the power of the light source which gives off energy. The intensity I, can be high, but it may not produce any work (or energy) at that point unless you have something to absorb that light. The more value of I, the more energy you can absorb if all other parameters hold.

3. Nov 9, 2007

### pardesi

actually can u give a mathematical interpretation

4. Nov 9, 2007

### Staff: Mentor

Are you asking about the units? Usually when we talk about the "intensity" of light, we're referring to its irradiance, which is the amount of energy it carries through a surface perpendicular to the propagation vector $\vec k$, per unit area, per unit time. It's usually measured in joules/(m^2 . sec) = watts / m^2.

Sunlight at the earth's surface has an irradiance of about 1400 W/m^2. If you had a 1-m^2 solar panel that could capture light energy with 100% efficiency, and oriented it so it directly faces the sun, it would produce about 1400 joules of energy per second, i.e. 1400 W.

Last edited: Nov 9, 2007
5. Nov 9, 2007

### pardesi

actually i was solving a problem and i was asked the power at a point .i know the intensity at that point how can i find the power

6. Nov 9, 2007

### Staff: Mentor

What units does the intensity have, in your problem?

7. Nov 9, 2007

### pardesi

it has $\frac{W}{m^{2}}$

8. Nov 9, 2007

### Staff: Mentor

Then that's all you can say about the power, "at a point." If you want a number of watts (not watts/m^2), you need to specify a particular surface through which the light falls on, or passes through, and multiply by the area of that surface.

9. Nov 9, 2007

### pardesi

yes that's what i thought too that's why i posted so