Research done by Google demonstrates that the use of shadows can affect the accessibility of a page for low-vision users. 

- Shadows and elevation can impact user identification of elements. Other attributes that can affect this include font characteristics, elevation, color, layout among surrounding elements, and context of use. 
- Speed and efficiency of element identification can be increased with the use of shadows. 
- Shadows and outlines can help a user to determine if an element can be interacted with.

University of Michigan - Ann Arbor

"Shadows provide cues about depth, direction of movement, and surface edges. A surface’s shadow is determined by its elevation and relationship to other surfaces."

Shadows

Material Design Documentation

The way shadows are used throughout a product should be consistent, so as to accurately convey the degree of elevation between surfaces. How large or small a shadow is, reflects the element's elevation. Shadows can also demonstrate selection.

RULE: The larger the shadow, the higher the elevation, and vise versa.

Usage

"Shadows provide useful cues about a surface’s direction of movement and whether the distance between surfaces is increasing or decreasing." 
- When an element changes elevation, its shadow should change. 
- When an element is changing shape or size, but elevation does not change, the shadow should stay the same. 

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