Properties of Light and Spectrophotometry

A.P. Biology

Mr. Knowles

Spectrophotometry

What is light?

Light is visible energy!

Some animals can see other parts of the EM spectrum!

Who can “see” IR?

The Story of Pit Vipers!

Video: Discovery- Viper

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pli tu

de

How does wavelength compare to energy?

An Application: Video- The Blue Planet: The Deep

Relationship Between Wavelength and Energy

• Wavelength and Energy Level are inversely related.

• The shorter wavelength, higher energy level.

Light and Wavelength • Length of a light ray- wavelength.

• Usually represented by the Greek letter lambda ().

• Sun produces a continuous spectrum of EM radiation.

• Visible light is from about = 420 nm (blue) to 680 nm (red).

• Ultraviolet Light is = 100-400 nm.

What happens when light strikes an object (molecule)?

• Reflection

• Refraction

• Transmission

• Absorption

Absorption

• Absorption- when light rays do not pass through nor do they bounce back, but are used; when light rays are neither transmitted nor reflected, they are absorbed.

How do molecules absorb light?

• Molecules have various chemical bonds with electrons in certain energy states.

• Some of these electrons become excited by the light energy and absorb it.

• Because of the electron’s specific energy state, these electrons can only absorb certain wavelengths.

What determines absorbance of a molecule?

• Chemical nature – bond type, atomic structure of molecule.

• Concentration of Molecule – how much is present.

Absorbance is a Molecular Fingerprint!

• Every molecule has its own unique absorption spectrum – ability to absorb certain wavelengths (energy) better than others.

• Can be used to identify molecules.

Abs. Max of b = 453 nm

Abs. Max of a = 430 nm

Eyes are not created equal!

The Story of the Amazing Owl Eye!

Video: Built for the Kill-Night

What would happen if an eye had more cone cells? All the better to see your

color, my dear!

Video: Discovery- How They Hunt (Mantis Shrimp Eye)

Or

Discovery- Extreme Senses

What happens to the energy of absorbed light?

• Energy from light rays can be used.

–Green leaf absorbs all wavelengths (except green) for use in photosynthesis.

• Energy from absorbed light is transformed into heat.

–Ex. Black car on a hot day.

Transmission

• Transmission- passage of light through an object.

• Opaque objects cannot transmit any light.

How do molecules transmit light?

• Wavelengths of light that cannot be absorbed are transmitted through the molecules with no change.

• Absorption and Transmission are inversely related.

What happens to the energy of absorbed light?

• Absorbed light rays are changed into longer wavelengths and reemitted- luminescence

• Bacteria and other organisms can bioluminesce

Reflection

• Reflection- particular wavelengths strike an object and bounce back

• Reflected wavelengths give objects color.

Refraction• Refraction- bending of light as it

passes from one medium to another of a different density.

• Index of Refraction- a measure of the speed at which light passes through a material.