Question

If the atmosphere is like a greenhouse, what parts of it function as the "glass"?

Answer 1

Greenhouse effect is basically nothing but heat/energy that has reached the earth's surface is re-trapped inside the earth's atmosphere. 

Element of earth that can be compared to the 'glass' analogy in greenhouse are 'Gases' within earth's atmosphere. These gases are called as 'Greenhouse gas'. 

Greenhouse gases are ozone, carbon dioxide, water vapor, nitrous oxide, and methane. That's the reason that increasing concentration of CO2 gas is worrisome as it contributes to higher heat trapping causing global warming. 

Answer 2

The atmospheric gases, created due to pollution, and suspended particulate matters create a cover that is quite like a glass cover of greenhouses. Technically, therefore, the effect of this is called the 'greenhouse effect'.