Key Areas covered Irradiance and the inverse square

Key Areas covered • Irradiance and the inverse square law. • Irradiance is power per unit area. • The relationship between irradiance and distance from a point light source. • Line and continuous emission spectra, absorption spectra and energy level transitions.

What we will do today: • State how we can calculate the irradiance of radiation. • State how intensity and distance of a light source are related. • Carry out calculations on the above.

Irradiance (intensity)

Irradiance • • • When radiation falls on a surface, the irradiance, I, is defined as the power per unit area: I= P A I – intensity (Wm-2) [or lux] P – power (W) A – area (m 2) (Irradiance is referred to as intensity in some past papers)

Experimentally proving the inverse square law • A light meter can measure the intensity of light from a bulb in a darkened room at various distances over a metre.

Experimentally proving the inverse square law • Measuring irradiance levels at different distances and plotting a graph of results can be used to confirm the inverse square law.

Irradiance and distance • • • As you move further away from a point source, the intensity of radiation decreases. The relationship between intensity (I) and distance (d) can be shown to follow an inverse square law: I α 1 in data book as I = k d 2 Therefore: I 1 d 12 = I 2 d 22 *This is only the case for a point source of light i. e. a lamp not a laser (or a torch)

Example • A lamp shines on a screen of area 2. 5 m 2, which is 3 m away. The intensity at the screen is 0. 01 Wm-2. a) Calculate the power of the incident beam. b) If the screen is moved to a distance of 1. 5 m from the lamp what would the new intensity be? a) P = I A = 0. 01 x 2. 5 = 0. 025 W b) I 1 d 12 = I 2 d 22 so I 2 = I 1 d 12 d 22 I 2 = (0. 01 x 9) 2. 25 I 2 = 0. 04 Wm-2

2001 Qu: 18 (repeated in 2011!)

2005 Qu: 17

2005 Qu: 13

2003 Qu: 17

2008 Qu: 28

Past Paper Questions Revised Higher

Revised Specimen Paper A

2012 Qu: 17