# Whiteboard Warmup! A glass lens of refractive index n = 1.6 has a focal length of 30 cm while in air. What would happen to the focal length of the lens.

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Whiteboard Warmup! A glass lens of refractive index n = 1.6 has a focal length of 30 cm while in air. What would happen to the focal length of the lens if… a) A material with a higher index of refraction were used to construct the lens instead? b) The entire lens were immersed in water (n = 1.33)? Draw ray diagrams to support your answer. Use Snell’s Law!

A higher index of refraction will cause light to refract more at both sides of the lens. This will make the focal length shorter. Immersing the lens in water will cause light to refract less at both sides of the lens. This will make the focal length longer.

The Wave Model of Light

The Ray Model has shortcomings Although it correctly predicts reflection and refraction of light, it cannot explain Dispersion of light (different colors refract by different amounts) Double slit interference! – Completely killed the Ray Model The Ray Model also cannot explain why light refracts! Another, better model is necessary.

Dispersion The phenomenon by which different frequencies of light refract by different amounts. Each frequency of light has a specific index of refraction in any given material. This causes white light to separate into its color components when incident on a triangular prism.

Dispersion of Light The higher the frequency, the more the light will refract. Violet refracts the most – red refracts the least.

Disperse to Your Whiteboards! A beam of white light in crows glass is incident upon air, as shown below. The refractive index of red light in the glass is 1.67. The refractive index of violet light in the glass is 1.73. The refractive index of both colors is approximately 1.00 in air. Determine the angle that separates the red and violet portions of the white light once it enters the air. 10° θ

A beam of white light is incident on a triangular glass prism with an index of refraction of about 1.5 for visible light, producing a spectrum. Initially, the prism is in a glass aquarium filled with air, as shown above. If the aquarium is filled with water with an index of refraction of 1.3, which of the following is true? (A) No spectrum is produced. (B) A spectrum is produced, but the deviation of the beam is opposite to that in air. (C) The positions of red and violet are reversed in the spectrum. (D) The spectrum produced has greater separation between red and violet than that produced in air. (E) The spectrum produced has less separation between red and violet than that produced in air.

The Wave Model of Light Light is a transverse electromagnetic wave! – It is composed of perpendicular electric and magnetic fields that propagate one another. Light waves can constructively and destructively interfere with one another. Light waves obey v = λf Light waves propagate according to Huygens’ Principle.

Crossed, oscillating electric and magnetic fields will propagate indefinitely and without loss of energy at speed c through a vacuum.

The Electromagnetic Spectrum! All of these frequencies of light travel at speed c in a vacuum (3 x 10 8 m/s).

A mnemonic to help you remember the spectrum! (in order of increasing frequency) Rattlesnakes May Inject Venom Upon eXtreme aGitation Radio Microwave Infrared Visible Ultraviolet X-ray Gamma

Human eyes are only able to detect light of wavelength 480-720 nm. That is why this is called the visible range of the spectrum. The wavelength that we perceive as red is about 480 nm. The wavelength that we perceive as violet is about 720 nm. Different animals are able to detect different ranges of EM waves! The image on the right shows the ultraviolet light given off by a dandelion. Bees and other insects have eyes that are capable of detecting UV light!

Why are we able to detect 480 – 720 nm electromagnetic waves with our eyes? That is the peak range of wavelengths emitted by our Sun!!!

What happens to the speed and the wavelength of light as it crosses the boundary in going from air into water? Speed Wavelength (A) Increases Remains the same (B) Remains the same Decreases (C) Remains the same Remains the same (D) Decreases Increases (E) Decreases Decreases Quick Conceptual Whiteboard Review!

Frequency of a wave does not change upon entering a new medium! The frequency of an EM wave governs how much energy it carries. Frequency is a property of the wave, and is set once the wave is produced. Wave speed and wavelength will change inversely upon entering a new medium!

The Doppler Effect also applies to light! If a source of light is moving toward an observer, the light that the observer receives will have a higher frequency and shorter wavelength than would normally be received! This is called blueshift. (Light is shifted toward the blue end of the spectrum – higher frequency) If a source of light is moving away from an observer, the received light will have a lower frequency and longer wavelength than normal! This is called redshift. (Light is shifted toward the red end of the spectrum – lower frequency)

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