Lecture 4: Light manipulation by biological systems

Question 1

What’s the Brewster angle ?

Answer 1

When the angle between the reflected and refracted beams is 90°
Polarization is total.

Question 2

What is the purpose of optical fibers ?

Answer 2

They operate using total internal reflection. The incidence angle must respect
sin(teta) <= sqrt((n2)^2 -(n1)^2)
- Designed to transfer information.
- Transmission with optical fibers is faster and superior to copper cables
- Interior material must have a larger index of refraction relative to the cladding (material wrapped around the glass).
- Many types: step index, graded index, single mode, photonic crystal

Question 3

What’s the main difference between step index fiber and graded index fiber ?

Answer 3

-Step index fiber has a uniform refractive index throughout the core, but with a sharp change at the core-cladding boundary. This causes light to enter the cladding and be reflected back into the core at this interface, leading to light being trapped within the core.

-graded index fiber, on the other hand, has a refractive index that varies continuously from the center of the core to the cladding.
The refractive index decreases as the distance from the center of the core increases. This gradual change in refractive index helps to mitigate the effects of multimode dispersion, which is a major limitation of step index fibers.

Question 4

What is structural coloration ?
To what optical components can the butterflies’ wings be associated with ?

Answer 4

Mechanim by which colour emerges from structure rather than as a result of absorption/emission. Color is generated by constructive and destructive interference resulting from light reflecting off the object.

• Grating

Question 5

What are metamaterials ?

Answer 5

• Special materials that derive function from their structure.
• Engineering applications use metamaterials to mimic nature (solar energy management, sensors and communication).
• Superhydrophobic surfaces can be designed to mimic elements in nature (e.g. lotus leaf).

Question 6

How do diatoms manipulate light ?

Answer 6

They use structural features acting as lenses to use reflected light for photosynthesis (convert CO2 into glucose, releasing oxygen). By-product is structural coloration.

Question 7

How does Venus’ flower basket use optical fibers to manipulate light ?

Answer 7

• The sponge’s skeleton is made up of silica-based fibers that act as optical waveguides, guiding light through the interior of the sponge.
• The optical fibers in the Venus’ flower basket are arranged in a hexagonal lattice pattern and are interconnected at each junction to form a highly interconnected network of fibers. This network of fibers allows light to be redirected, scattered, and absorbed within the sponge, which helps to create a unique light-manipulating system.
• The optical fibers serve various purposes such as protection, signaling, and attracting food.

Question 8

How can cephalopods use their cells to act as polarizers?

Answer 8

They have specialized skin cells called chromatophores that contain pigment granules that can be aligned in specific directions to act like polarizers. By controlling the alignment of the pigment granules in these chromatophores, cephalopods can selectively absorb or reflect light with a particular polarization direction, allowing them to control the intensity and color of the light that reaches their skin.

Question 9

How can pinhole vision be use to certain organims’ advantages ?

Answer 9

• Pit viper snakes : for detection and imaging of radiant heat
• Giant clam: eliminate the need to blink, allowing faster reactions
• Nautilus: 2 elliptical pinhole eyes that provide better resolution.

Question 10

What is the advantage of bifocal eyes ?

Answer 10

They allow for different focal planes.