<h2>
Answer</h2>
The physical state of the elements depends upon the <u>attraction forces </u>and their <u>kinetic energy</u>.
<h2>
Explanation</h2>
The elements or substances are fixed with each other with the help of different chemical forces including ionic bonding, covalent bonding, H- bonding etc. The strength of these forces is also one of the factors that affect their physical natures. For example, covalent or ionic bonds are the strongest bonds than all other bonds and metals that contain these forces are mostly in solid form. The kinetic motion of electrons in the element also affects the physical state of the element and potential of bonding.
The minimum speed of the particle is the Speed of light in glass is c/μ=2×108m/s.
<h3>Why is the refractive index important?</h3>
The higher the refractive index the slower the light travels, which causes a correspondingly increased change in the direction of the light within the material. What this means for lenses is that a higher refractive index material can bend the light more and allow the profile of the lens to be lower.
Refractive index values are usually determined at standard temperature. A higher temperature means the liquid becomes less dense and less viscous, causing light to travel faster in the medium.
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Crystalline crystals have sharp, well-defined melting points. Amorphous Solids don't have melting points.
Answer:
Explanation:
The relationship between angle and wavelength for maxima and minima in Young's double slit experiment is given by
For constructive interference

For Destructive interference

where 

m=order of maxima and minima
for second order maxima i.e. 
For smallest separation taking 



You want to draw a free body diagram of the forces on the sled in the horizontal x-direction.
If you visualize the system in an x-y coordinate plane, the force along the x-direction is the angle it makes with the x-axis multiples by the force.
The angle made with the x-axis is cosine of the angle theta.
Please see picture attached.