Question

Theory

a) How can you move a spacecraft just with solar radiation?

b) Explain how nanoparticle sunscreens can be transparent to the human eye, but still protect the skin.

c) What are the two main ways that nanoparticle sunscreens containing TiO2 or ZnO can protect your skin from ultraviolet radiation? [Note: the UV wavelengths of most concern are labelled UV-A (400-315nm) and UV-B (315-280nm)].

d) What is the largest size of TiO2 nanoparticle that could be used to make a transparent sunscreen (before to apply it)? [Hint: use the size parameter and wavelength of 500 nm]

e) What other factors would you need to consider before reducing the particle size to make a sunscreen transparent?

f) What colour would be the sky at noon (assuming normal incidence of the sun rays) if the atmosphere was made by 10 um particles?

g) Explain why is possible to do sensing with plasmonic nanoparticles.

h) Explain the working principle of a microwave oven. Would it still work if you doubled the wavelength of the exciting radiation?

i) Is it possible to detect spherical particles in air with a diameter of 1nm with the IR light? Comment the answer.

j) Is the Rayleigh scattering at λ=400 nm greater or smaller than the one at λ=700 nm for equal incident intensity of light? Explain the answer.

Answer 1

a) We can move a spacecraft with solar radiation and this is called Solar Sailing. In Solar Sailing, the spacecraft is moved by the light emitted by the Sun.

For moving spacecraft sunlight which is reflected by a mirror hence the mirror experiences a pressure across its surface called Solar Radiation pressure. This pressure is enough to move a spacecraft in vacuum of space.

b) When we rub the sunscreen onto our skin then it reflect visible light, but when the particle size are in nanometer range then it reflect the ultra violet rays of the sun hence protect our skin from skin cancer.

c) The products which we consume in our daily life contains nanoparticles. Sunscreens contain titanium dioxide and zinc oxide which reflrct or absorb cancer causing ultra violet rays.

Titanium dioxide nanoparticles posses different physicochemical properties which reflect the harmful ultraviolet radiations of the sun and protect our skin.

It has been found in a dermal study ,whether vivo or vitro report that these nanoparticles donot penetrate the stratum comeum.

d)The largest size of Titanium dioxide nanoparticles that can be used to make transparent sunscreen is 25-50 nanometer wide.